I tend to end up explaining these things a lot to guildies and friends so I figured I might as well put them down on paper, so to speak. These will primarily be from a Heroic perspective and we're gonna start with my least favourite, and the one that gives many grief it seems: Grim Batol.
Ahhhhhh.... I hate this place with a passion. It's the trash that makes it tedious largely because it's designed around the premise that you're going to have 5 people who know the path to bomb for the drakes (Left side to the first bridge, right side to the first boss, screw the troggs unless you need the quest - which you don't have to get done in one trip BTW - before opening up again on the left side mobs as you enter the next atrium. Then, down the right side to the first bridge, back to the left to the last boss and whatever you can throw on the right as well because that's the path to the last), and the reality is that this will very rarely be the case. So, you end up with a ton of trash pulls between bosses and a really long, tiring, instance. That said, lets hit up the bosses:
General Umbriss has two main attacks of worry, Blitz and Ground Slam.
Blitz prefers ranged targets, but will pick targets in melee range if no one maintains their distance, and will be announced. When this happens the person being targeted will find themselves standing in one of those super happy "Really bad thing is going to happen here!" circles. Exit the circle. Quickly. Do not finish your cast or try to get in one last lick; on heroic this ability one shots.
Ground Slam is an announced Shockwave attack that does significant damage and stuns anyone it hits. Tanks and people in melee range should endeavour to move out of it's way - especially once Umbris enrages in the last phase of the fight and this ability can quickly become fatal to anyone already taking damage like the Tank. Handily, when this attack is announced Umbriss is going to turn and face the person he's going to cast towards. At this point, get out of his immediate frontal arc if you're in melee range to avoid the attack entirely.
The last mechanic on this fight to worry about are the Troggs that run in periodically. There will be 3 lessor Troggs who can be safely tanked by anyone in the room (although you still want to keep them off the healer), and one big purple Malignant Trogg who hits fairly hard and, if you're going to kill him, must be killed away from the boss and the other Troggs. This is because the Malignant Trogg is infected by a disease that will spread to anyone in his immediate area when he dies and cause Umbris or the other Troggs to enrage while putting a DoT on any players it hits as well. For this reason, it's best to have Ranged kite and kill him off to the side. Note, however, I say "if you're going to kill him" because I've been told you can CC the first spawn of this mob for the whole fight and then never get another but haven't had a chance to test this myself yet. If you've got a good CC crew, I'd do this instead and then your Tank and/or DPS just need to keep picking up/exploding the little Troggs.
Moving on, the second boss you face will be Forgemaster Throngus.
You only need to clear the first part of this room - the patting Troggs, the two Dragonkin, and the big Gronn and his three casters. Note, you want to CC those casters as much as possible as they chain spam DoTs across the entire party and will quickly overwhelm healers if you let all three have their way.
Once that's done, Throngus is pretty straight forward as long as you understand what's going on in his three phases. While they can come at random, you can expect to have Throngus "equip" three different gear combinations. When it announces that he's equipped his Two Hand Mace, the Tank needs to turn and run, not backpedal, run from Throngus as any hit will one shot the Tank. Throngus will be "encumbered" while doing this so running away is not difficult and the onus really falls on the DPS to make sure they don't jump past the Tank on threat if this happens early in the fight. Throngus will be leaving a trail of fire as he stumbles along during this phase, so don't stand in it, and periodically spearing random players on his mace that will need some healing before he throws them back to the ground.
Next up, when it announces Throngus has equipped his Phalanx and Shield he's going to start breathing fire at a random target and "Archers" are going to start raining flaming arrows on the party for extra AoE damage. The best way to deal with this phase is to bunch up on Throngus and stay in his rear arc as he picks a target and breathes fire in that direction for about 20 seconds so your Healer can make best use of his AoE tools.
Last in our list, though never necessarily in his, Throngus will equip Twin Swords. This is a massive damage phase on the Tank so the Tank and Healer should pop/rotate cooldowns to help keep him up. During this phase Throngus will also be stacking a Fire DoT on the Tank: the Healer should dispell this and anyone who can help him should consider doing so because, if left to it's own devices, the combination of this DoT as it stacks and the Melee damage coming in will make short work of the Tank.
Finally, throughout all three of those phases Throngus will periodically cause sections of the roof to "collapse" in a rain of shards onto the floor below; as usual, don't stand in the void zone.
Once Throngus is dead, you can skip the rest of the room by porting out of the instance, back in, and taking the Red Drakes at the front of the instance to the next wing but, and this is a very important but, you must be sure to send the tank first and that he knows to double back towards the entrance back into Throngus's room if necessary because there's a pack of patrolling Troggs in the far hallway where you'll be landing as well and the Drakes will split the party between the far and near ends of that connecting hallway when they land. Going ahead of the tank is a good way to get nuked by these Troggs.
Our third boss is the RNG joy that is Drahga Shadowburner. This is technically a three phase fight though the first and last phases are functionally burns; it's the middle that will make you cry if you don't control it well.
You're going to open with the Tank pulling Drahga and everyone DPS'ing him to about 20% health when he will jump off the edge of his platform and get on the Twilight Dragon Valiona; at which point phase two begins. During this first phase and continuing into the second Drahga will periodically be screaming "Incinerate Them Minion" and summoning Flaming Sprit's from fiery circles on the ground. It's very important that three things happen WRT To these:
1: The Flaming Sprit can pick anyone in the party as it's target so everyone - including the Tank - needs to put some distance between themselves and the spawn circle.
2: No one should get hasty and charge the circle until the Flaming Sprit picks a target - indicated by a fire beam between the Spirit and the target - because the Spirits are proximity bombs. If they reach you on heroic they explode and you, as well as anyone unfortunate enough to be nearby, are dead. So, again, stay away from the circle until the mob picks a target.
3: The Flaming Sprit can be slowed or rooted so any of these should be on it the split second it solidifies in the fire circle to make it easier to kite by it's target.
All that said, as soon as those conditions are met, untargetted DPS should make these explode as quickly as possible once it's safe to do so.
Where doing this can get fun is in phase 2 when Valiona joins the fight. Firstly, the large dragon tends to crowd sight-lines - especially for melee/tanks - so remember: controlling the mob that can one shot you, your Healer or your Tank is more important than DPS'ing the boss. Pay attention to the emotes and start looking for your spawn point as soon as possible. Secondly, Valiona brings a couple abilities of her own to the table; one of which can prove especially deadly in combination with the fire Elemental if not controlled properly.
The first ability is that Valiona will periodically spawn a large void zone directly underneath her which the Tank will need to kite her out of; this is not particularly problematic and easy enough to avoid. The second, and the one that will give you the most problems, is that Valiona will periodically turn, face a random target, issue a raid warning, and then breathe fire in a very large frontal half circle for 5-6 seconds that does extreme damage if you don't use the time between the announce and cast to get in her rear arc. Because of this, it is unwise as Ranged/Melee/or Heals to get too far away from the dragon as finding yourself "out in the middle of nowhere" on this fight is a good way to end up dead when she breathes. It's this ability the Tank also needs to manage by keeping Valiona relatively on the centreline of the room because you don't want to find yourself with the tank up against the wall and Valiona breathing along the rest of the room for two reasons - firstly that it means everyone has to stack in a small area to avoid the attack and secondly because it leaves no room to kite for those fun times when the Flaming Spirit spawns in near proximity to the breath phase which is a good way to find yourself dead.
If you're good at dodging the breath and destroying the adds instead of them you, at 20-25% Valiona will tell Drahga to do his own work, throw him off, and leave him to be killed by you during the third burn phase. One final Healer note: there's some periodic party damage on this fight because Drahga will be casting spells at random targets throughout the battle independent of the threat table.
Last up is Erudax which is primarily a DPS and execution check. He has three mechanics of note to himself. The first one would be Binding Shadows. Erudax will cast this at individual party members and will face them while doing so before casting the spell towards where they were - this is similar to the Shadow Crash ability Faceless Ones used in LK only in this case it applies a DoT and Root to anyone it hits so spread out. The good rule of thumb on this fight is that, if Erudax is facing you and you're not the tank, you'd best "Waggle" to see if he keeps following you, and, if he does, keep moving to avoid the attack.
You particularly don't want to get rooted because of his second periodic ability: Shadow Gale. When it announces he's beginning to cast this you will see a swirling purple "hurricane eye" appear in the room. Everyone must be in the middle of this circle before the cast starts or else they will take large amounts of damage from the AoE Erudax will be channeling. To help do this, the Tank should keep Erudax in the middle of the centre circle in the room - returning there when Erudax executes his knockback ability - because these "eyes" will spawn around that inner circle and it's very important that Erudax be next to them for Melee during the Gale Phase. During this phase the boss takes 100% more damage so, if the Melee can't hit him, you're losing substantial DPS. This is also a good time to use Heroism/Bloodlust/Timewarp/etc.
Immediately following the Shadow Gale Erudax will summon two Faceless Corruptors who will enter from the staircase entrance to the room and begin making their way towards the eggs. As soon as the Gale ends, the DPS needs to break from the boss as fast as possible and head for the staircase to get, and keep, slows on these mobs - the "slower" the slow the better so if you can talent/glyph/etc this is the time to use it - to increase the time they have to kill them. If the mobs are not killed before they reach the eggs they begin hatching them which does two things: adds stacking periodic damage to the party and heals for the boss. You do not want these adds to reach the eggs. The usual way of dealing with these adds is to assign two DPS to one mob and the DPS with the "slowest" slow to the other with the intent being that those first two can use their slow to burn their mob quickly and move to help the third with theirs when the first dies because the third will be moving slower towards the eggs. This is the key phase of the fight and your DPS need to have the attitude that the mobs must be slowed and killed ASAP; any lingering on the boss will simply get you all killed.
Cycle through these three abilities 3-4 times and you should find you've got Erudax down and you're outta here...