The first boss is a 3 in 1 encounter.   You'll fight 1 after another in succession.  

Each of the bosses are individually easy.   The trick is probably more that it's an endurace thing, there's zero break inbetween bosses.

Gormok the Impaler

Abilities:

• Impale - Inflicts 1,750 to 2,250 (25-Players: 2,188 to 2,812) Physical damage per application every 2 seconds, for 30 (25-Players: 40) seconds.
• Staggering Stomp - Inflicts 7,800 to 8,200 (25-Players: 11,700 to 12,300) Physical damage to all enemies within 15 yards and interrupts spellcasting for 8 seconds.
• Rising Anger - Increases all damage dealt by 15%. Applied every time Gormok throws a Snobold.

• Snobolds - Throws a Snobold Vassal at a random player. (HP:  134,810 - 539,240)  Gormok has 4 (25-Players: 5) Snobolds to throw. They will use the following abilities at their target (who will receive Snobolled! debuff) and other players nearby:

  1. Batter - Inflicts 75% of normal melee damage, interrupting spellcasting for 5 seconds.
  2. Fire Bomb - Inflicts 4,813 to 6,187 Fire damage every 1 second to all enemies within 8 yards of the target.
  3. Head Crack - Stuns an enemy for 2 sec.

The Fight:

A simple tank and spank but can hit the tank hard.   You'll need 2 tanks to trade taunt when the Impale bleed stacks too high, say every 3-4 of them.   He is also disarmable, which prevents impale.   Hand of Protection also removes the debuff.

If you don't have to be in melee, stay 15+ yards away.   He has that stomp that can interrupt your cast and damage you, so stay out if you can.

There are Snobol Vassals that spawn, they don't hit for much but can interrupt your casters.   They throw Fire Bombs which just leaves patches of fire to get out of.  They are more likely to focus on ranged, prob global aggro but occasionally hang out in the melee.  All your ranged dps should burn them down ASAP.

At the end of the fight, don't stop healing the tanks, they probably still got bleeds as you head into the next fight (starts right away).

Hard Mode:

• Impale - Inflicts 3,500 to 4,500 (25-Players: 3,938 to 5,062) Physical damage per application every 2 seconds, for 30 (25-Players: 45) seconds.
• Staggering Stomp - Inflicts 9,263 to 9,737 (25-Players: 11,700 to 12,300) Physical damage to all enemies within 15 yards and interrupts spellcasting for 8 seconds

It's basically the same fight but everything hits harder.   Impale also lasts 45 seconds now, and he cannot be disarmed so tanks have to eat it.    That means you need 2-3 tanks taking the impale.     We chose 2 tanks and let it stack to 4 so we'd have the dps to kill him before the Worms appear.    Use defensive CDs, remember he hits harder at the end, and BoPs remove the bleed.

Fire bombs also leave a fire dot that hurts, so more emphasis on not failing it.   The Rising Anger also becomes more of an issue as more Snobalds arrive.   In 10 man, there was no problem with ranged just killing them off as they appear.    Prioritize the snobalds on healers, casters, then the rest (melee/hunters).

There's also maybe a bug, or maybe this is the intention, normally there's 4 Snobalds, so 60% total increase in damage at the end.   But if a guy with a Snobald dies, or if you do something to knock off a Snobald like Ice Block or whatever, that Snobald becomes free and runs loose with normal threat rules, and Gormok throws out another Snobald (a 5th) which also adds another Rising Anger (75%).     

Also, you apparently have about a 2.5 min timer or around there before the next one shows up, so it's a bit of a dps race.

Achievements:

Upper Back Pain:  Defeat Icehowl while at least 2 / 4 Snobolds remain alive in 10 / 25-player mode.

This is the only achievement related to him.   Keep Snobolds around, and I suspect just let them annoy you the rest of the fight.  

Acidmaw and Dreadscale

Abilities:

Acidmaw:

Mobile:

• Acidic Spew - Inflicts 2,775 to 3,225 (25-Players: 3,700 to 4,300) Nature damage every 0.25 seconds for 2.50 seconds, to enemies in frontal cone.
• Paralytic Bite - Inflicts 7,863 to 9,137 (25-Players: 11,100 to 12,900) Nature damage to its target and injects them with a Paralytic Toxin.
  

Stationary:

• Acid Spit - Inflicts 5,088 to 5,912 (25-Players: 6,475 to 7,525) Nature damage to a random target.
• Paralytic Spray - Inflicts 6,938 to 8,062 Nature damage to its target and other players within 10 yards, injecting them with a Paralytic Toxin.
• Sweep - Inflicts 6,938 to 8,062 Physical damage to enemies within 15 yards and knocks them back.

Both:

• Paralytic Toxin - Inflicts 3,000 Nature damage and reduces movement speed by 10% every 2 seconds. Lasts for 60 seconds, or until removed by Burning Bile.
• Enrage - Increases all damage dealt by 50%. Used when Dreadscale dies.

Dreadscale:

Mobile:

• Burning Bite - Inflicts 7,863 to 9,137 (25-Players: 12,950 to 15,050) Fire damage to its target and coats them with Burning Bile.
• Molten Spew - Inflicts 2,775 to 3,225 Fire damage every 0.25 seconds for 2.5 seconds, to enemies in frontal cone.

Stationary:

• Fire Spit - Inflicts 5,088 to 5,912 (25-Players: 6,475 to 7,525) Fire damage to its target.
• Burning Spray - Inflicts 6,938 to 8,062 Fire damage to a target and other players within 10 yards, coating them with Burning Bile.
  

Both:

• Burning Bile - Inflicts 3,238 to 3,762 Fire damage every 2 seconds to allies within 10 yards. Removes Paralytic Toxin. Lasts for 24 seconds.
• Enrage - Increases all damage dealt by 50%. Used when Acidmaw dies.

Whichever is Mobile:

Slime Pool - Spawns Slime Pool underneath the jormungar's body. The Slime Pool grows in size for 30 seconds and inflicts ~2,000 Nature damage every 1 second to players standing in it.

The Fight: 

The start of the fight has Dreadscale coming out on the right side, while Acidmaw comes out from the ground on the left side.    Acidmaw will be rooted to the ground for now.    The 2 will occasionally burrow under the ground and reset aggro and swap roles, i.e. Acidmaw becomes movable, while Dreadscale is rooted, and vice versa.

The one that's rooted will be casting spells fast and will also occasionally Sweep the melee knocking them away.    A ranged tank can tank him at this point, but a proper melee tank is prob still good and just soak the damage.    The one that's movable will be dropping Slime Pools, ala Grobbulus style, though not as often.   Just move as it occurs.

Acidmaw, will be launching Paralytic Spray, it's a green ball of slime that goes up and splashes on to the ground.   Whoever's hit will be afflicted by Paralytic Poison.    They will eventually slow down to a halt, we decided to have those move to the middle.

Dreadscale will be doing Burning Spray to his target, which is also a fire aoe splash that gives people the Burning Bile debuff.   That will cause the player to do fire damage to everyone around him.   Have to spread out to avoid hitting everyone in raid.    But the fire damage also cleanses the Paralytic Poison, so that guy has to weave around the raid cleansing those people.    

What we wound up doing was assign 1-2 people to stand with the Dreadscale tank, take the Burning Bile debuff.   Then go around cleansing the raid.    Though they should all be around the center, but guess that might not always happen.

That's pretty much it to the fight.   Recommended you kill Acidmaw before Dreadscale but it's not neccessary.  The only thing to cure the Poison is the Burning Bile, so if you run out of Bile, you may be screwed on poison for the next fight.     Well, heal them and wait for debuff to wear off, and hope it's not a tank that's poison if you do that.

Also, one will enrage when the other dies.  So killing them close together probably most ideal.    But itt's not a lethal enrage, just zerg it how you feel like.

Hard Mode:

Acidmaw:

• Acidic Spew - Inflicts 3,700 to 4,300 (25-Players: 4,625 to 5,375) Nature damage every 0.25 seconds for 2.50 seconds, to enemies in frontal cone.
• Paralytic Bite - Inflicts 12,950 to 15,050 (25-Players: 18,500 to 21,500) Nature damage to its target and injects them with a Paralytic Toxin.
• Acid Spit - Inflicts 6,938 to 8,062 (25-Players: 10,175 to 11,825) Nature damage to a random target.
• Paralytic Spray - Inflicts 8,325 to 9,675 (25-Players: 12,950 to 15,050) Nature damage to its target and other players within 10 yards, injecting them with a Paralytic Toxin.
  
• Paralytic Toxin - Inflicts 3,000 Nature damage and reduces movement speed by 10% every 1 seconds. Lasts for 60 seconds, or until removed by Burning Bile.

Dreadscale:

• Burning Bite - Inflicts 11,110 to 12,900 (25-Players: 18,500 to 21,500) Fire damage to its target and coats them with Burning Bile.
• Molten Spew - Inflicts 3,700 to 4,700 (25-Players: 4,625 to 5,375) Fire damage every 0.25 seconds for 2.5 seconds, to enemies in frontal cone.
• Fire Spit - Inflicts 6,938 to 8,062 (25-Players: 10,175 to 11,825) Fire damage to its target.
• Burning Spray - Inflicts 8,325 to 9,675 (25-Players: 12,950 to 15,050) Fire damage to a target and other players within 10 yards, coating them with Burning Bile.
• Burning Bile - Inflicts 5,550 to 6,450 (25-Players: 8,325 to 9,675) Fire damage every 2 seconds to allies within 10 yards. Removes Paralytic Toxin. Lasts for 24 seconds.

Both:

• Sweep - Inflicts 8,325 to 9,675 (25-Players: 10,175 to 11,825) Physical damage to enemies within 15 yards and knocks them back.

From the looks of things, first off, Hard Mode means stuff hits harder.   It's probably more important to spread out since Burning Bile hits 2-3 times harder.   Also the Paralytic Poison appears to tick faster, 1 sec instead of 2, so your time is halved before you freeze in spot.   

In the 10 man, we did the fight like normal.   Melee on mobile worm always.    Ranged emphasis on killing Acidmaw first so that he should die before Dreadscale.

In 25 man, it was similar to normal as well.  Though due to us being melee heavy we put our 3 DKs on the stationary one always.   While only 5 of us were on the mobile one so Burning Biles and stuff doesn't rape 8 of us at once.    Also our Ret Pally did his Bubble Sac during it to help out.   

Anyways, we started with physical dps (hunters/melee) on Dreadscale with 2 tanks passing the bile.  A warlock tank with casters on Acidmaw.   We use a Demonology spec'd Warlock with this Felpuppy out.   Combining the spell damage reduction mitigation along with Soul Link, making him healable.   Also healers should be aware of his pet and keep that alive too, else all his mitigation goes away too.   So healers might have to set up a focus target for the pet.  This allowed our lock to only take 6.5K spell hits, rather than a physical tank taking 8-12K along with melee and sweep hits.    He tanks the rooted target, i.e Acidmaw first, then Dreadscale after first submerge.

When Acidmaw is mobile, try to kill him before he submerges again, or get it fairly close.   Have all ranged (incl hunters) keep on him.  This is also similarly timed about 2.5 mins before Icehowl shows up.    Goal is to get Dreadscale dead too before that.

Achievements:

Not One but Two Jormungar:  Defeat Acidmaw and Dreadscale within 10 seconds of each other in 10/25-player mode.

Split the dps a bit, make them die within 10 sec of each other.   Only this part of the encounter is needed, no need to even kill Icehowl yet.

Icehowl

Abilities:

• Ferocious Butt - Inflicts 60,125 to 69,875 (25-Players: 69,375 to 80,625) Physical damage to his target and stuns them for 2.5 (3 in 25 man) seconds
• Arctic Breath - Inflicts 3,000 (25-Players: 4,000) Frost damage every 1 second for 5 seconds to a random target and other players in a cone between the boss and his target. Stuns and silences for the duration of the spell.
• Whirl - Inflicts 6,938 to 8,062 (25-Players: 8,325 to 9,675) Physical damage to all players within 15 yards, and knocks them back.
• Massive Crash - Inflicts 7,400 to 8,600 (25-Players: 9,250 to 10,750) Physical damage to all players, stuns them for 7 seconds, and knocks them to walls of the Coliseum. Used immediately before a Charge. When the stun wears off, all players receive Surge of Adrenaline for 8 seconds.
• Surge of Adrenaline - Increases movement speed by 50%.
• Trample - Charges at the location of a chosen target. Inflicts 50,000 Physical damage to everybody in its path, gaining Frothing Rage if he hits a player. If nobody is hit, Icehowl is affected by Staggered Daze for 15 seconds.
• Staggered Daze - Increases all damage taken by 100% for 15 seconds. Stunned for the duration.
• Frothing Rage - Increases Physical damage dealt and attack speed by 50% for 15 seconds. Enrage effect.

The Fight:

Tank and spank for the most part.   He does an Arctic Breath, so spread out around so he doesn't freeze the entire raid in one go.   He'll occasionally knockback the melee, so don't be there unless you have to.

Now the fun trick to the fight is that he'll occasionally jump to the center, then slams everyone to the back wall stunning us.   He will then choose a target, watch raid warnings, jump to the opposite side, and charge his target.       The stun at this point will wear off and you'll get increased run speed.

Now, if he hits you,  and anyone in the path, you'll probably die, unless saved by CDs, and go into an enrage, which will start 1-2 shotting tanks unless CDs or a timely Tranq shot saves him.      But, if you take those few seconds and move out of the way so he hits nothing but wall, SMACK, he gets stunned for a while, no enrage.

That's all he does.  Heal through any damage.   Strafe to the side everytime he targets you (or anyone near you) and yell ole!    Kill kill and repeat.

Crash Mechanics:

Ok after some testing I think I can confirm how Icehowl's charge works.

Point 1:  He does not kill everyone along the path.

Point 2:  He kills in an aoe on the point of impact.

Point 3:  The point of impact is where the player is the moment he is targetted (not necessarily the wall).

Point 4:  If you are the target of the charge, you cannot touch Icehowl.  Everyone else can, you can not.  And where he hits you becomes the new point of impact.

Point 1 and 2 means you can run any direction you want away.

But you have to be aware of Point 3.   Due to the pillers/corners around the room, you'll notice if you hit those, you'll be dragged along the wall a bit.  So the point of impact is actually at the piller in front of you.    In this case, running forward is likely bad since you might be running towards the kill spot.

Point 3 also allows for some interesting play.    If you were to say Vanish / Killing Spree / Ice Block / Bubble / etc to avoid the crash, then run to the center where he is before he targets anyone, and if he chooses you.   His jump back and charge will stop right at the center (or how ever far you make it).    This, assuming you yourself can get away, is likely the best scenario cause the crash is happening away from everyone (who would be at the walls), so they don't need to avoid shit (as long as they know to not run to where you were).

Anyways, gist of it, if you keep your backs away from corners/pillers of the room, be wary of which direction to run.   If you're no where near one of those, you can probably run straight forward.   If someone is messing with the Crash and playing with in the center of the room, don't run to that spot.

In addition, Point 4, added kind of forces the strat back to strafing away.  But, only the target has to do that, everyone else can scatter whatever direction they want assuming the target won't fail it.

Hard Mode:

• Ferocious Butt - Inflicts 69,325 to 80,625 (25-Players: 83,250 to 96,750) Physical damage to his target and stuns them for 2.5 (3 in 25 man) seconds
• Arctic Breath - Inflicts 4,000 (25-Players: 6,000) Frost damage every 1 second for 5 seconds to a random target and other players in a cone between the boss and his target. Stuns and silences for the duration of the spell.
• Whirl - Inflicts 9,250 to 10,750 (25-Players: 12,950 to 15,050) Physical damage to all players within 15 yards, and knocks them back.
• Massive Crash - Inflicts 10,175 to 11,825 (25-Players: 16,188 to 18,812) Physical damage to all players, stuns them for 7 seconds, and knocks them to walls of the Coliseum. Used immediately before a Charge. When the stun wears off, all players receive Surge of Adrenaline for 8 seconds.
• Frothing Rage - Increases Physical damage dealt and attack speed by 75% (25-Players: 100%) for 15 seconds. Enrage effect.

Again he hits harder.   We decided to tank him near a wall, back to the wall, so sweep doesn't knock our heavy melee dps too far away.

Fan out around for Arctic Breaths and have Pallies ready to Hand of Freedom the target to free all of them.   Blink, ice block, etc works too.

WIth the recent nerfs, you can recover from a failed wall crash, but probably no more than once.   Is still tranq'able now, so hunters be ready in case.   Also use Aspect of the Pack to help the raid, don't worry about daze cause getting hit is failing it.

There is an enrage timer now, 3 mins.    It's actually very tight timer needing almost everyone alive.    This fight is your gear check, most guilds are killing him right at the end of the enrage timer.

Videos:

 25 man kill: