Usually regarded as the second Firelands boss, Beth'tilac is a giant metal-plated lava spider with hordes of various spider babies and a very fun SWAT-style zipline mechanic. (Alternatively: Ninja mechanic.) It's a relatively simple fight strategy-wise, and is very much a brute force fight - your raid has to efficiently handle the waves of adds, and then be able to survive the straightforward but brutal burn phase.
One of the most important mechanics (really, the only actual mechanic?) is that Beth'tilac spends the first phase of the fight atop her web, spraying the raid with moderate fire damage. The only way to stop this damage is to kill her Cinderweb Spinner adds, who leave behind a web-line which players can use to climb up and engage her in melee.
Meanwhile, the players who don't climb up have their hands full dealing with wave after wave of spider adds - heavy-hitting Cinderweb Drones that need to be tanked, the above-mentioned Spinners, which need to be killed to free up more web-lines for the "Up Team," and swarms of little Cinderweb Spiderlings that have to be burned down quickly.
After a short period of fighting the Up Team, Beth'tilac will force them to jump back down to avoid Smoldering Devastation. She'll then start to spray the raid with fire again. She'll repeat this entire process three times, then decide to get serious and descend to fight the raid for real. At this point (once all the adds are mopped up) the fight becomes an extremely straightforward burn phase. For us, anyway. Tanks and Healers have their hands full, but we just nuke Beth'tilac as hard as we can!
Your special class/spec responsibilities:
• Kill Cinderweb Spinners immediately for web lines
• Up: Avoid Meteors, Smoldering Devastation
• Down: Kill Cinderweb Spiderlings, then Drones
If you're going up
If you're one of the people chosen to climb up to and smack Beth'tilac around a bit, your top priority before climbing up is to murder the Cinderweb Spinners IMMEDIATELY, so that they drop their web-lines. They need to be taunted down, but they're very weak and hit like a wet noodle, so go ahead and taunt and crush them as soon as you see them.
Also, it is CRUCIAL that you let the Tank, then the Up Team Healer (in that order) climb up BEFORE YOU. If you get over-eager and hop on a web-line meant for them, you will die. (And usually wipe the raid.) Be very careful that they have gone up before you grab a line and join them.
Once up, our job is simple. Just dodge the falling meteors, don't fall through the holes the meteors leave, and jump down to avoid Smoldering Devastation. There's nothing to interrupt, no adds to kill. Just burn her as much as you can. When it's time to hop down, be sure to jump down the natural center hole (not a meteor hole), as the center hole will slow your fall with web strands.
If you're staying down
If you're one of the ones who stay down below, you're on add management! Once again, killing the Cinderweb Spinners for your Up Team is the top priority. Taunt them down, splatter them over the floor, and move on.
The other adds are the Cinderweb Drones (who have a frontal venom cone, so stay behind them), and the Cinderweb Spiderlings. It's easy to get confused about which of these three add types to worry about first, so just think of them as a priority system - kill the Spinners first, then deal with Spiderlings (unless you have other DPS handling them), then the Drones.
These little critter-sized spiders will periodically spawn (with a raid warning announcement) and make their way towards players, or a Drone if one is alive. If they reach a Drone, or if any are still alive when Beth'tilac descends, it is VERY BAD - the Drone/Beth will Consume them, healing and increasing damage dealt.
You can think of these little guys as Magmaw's Lava Parasites, round two. They have to be burned down quickly, and from a distance (they do bad things to players if they get near). Perfect opportunity for Chilblains-ed Howling Blasts! (You *do* have Chilblains, don't you?) The heavy damage and lengthy slows make us very valuable for handling these pests.
Even if you don't have Chilblains, or if your other DPS are handling the Spiderlings, keep an eye out and blast them with a mouseover Howling Blast if they start getting near. (You *do* have Howling Blast bound to a mouseover macro, don't you?)
Alternate Strategy: Up, Up, Down
In our raid, we have one DPS in the Up Team climb up like normal the first two times, but stay down below for the third wave. It is crucial that the adds are down before Beth descends, especially the Spiderlings, so having an additional DPS down below for the last add phase is quite helpful.
Think of it this way - any damage done to Beth'tilac before she descends is really just a bonus, and only serves to reduce the length/hardship of the burn phase. Now sure, this is important, and having that DPS up top might mean a few extra percent knocked off...but having a single Spiderling alive when Beth hops down will heal her for 10 PERCENT. That is a HUGE setback. Not to mention that the last Drone will most likely still be alive, and if your raid takes too long to finish it off, it could result in a wipe.
Army of the Dead
A good time to Army is when Beth'tilac has landed and all the adds are dead, ideally before Bloodlust. Don't do it on pull - Beth isn't even there to hurt, so it would be wasted, and the ghouls will probably just cause havoc with the adds.
There is a ton of unavoidable, predictable fire AoE damage in this fight - every time Beth'tilac is atop her web spraying the raid, and in the form of Ember Flares, which damage players on the same level as her. If you are up top, I would use Anti-Magic Shield once you're up to help out your healer, as the damage can be rough. Otherwise, I would AMS after each Smoldering Devastation, when you're waiting for Spinners to spawn and getting sprayed with fire from above. Finally, when Beth lands use AMS on cooldown to reduce the Ember Flare damage.
Beth'tilac drops Arachnaflame Treads, which are very nice for Death Knights - gotta love Red sockets with +Strength bonuses!