(Paraphrased from 25man.com)
Split the raid into two groups -
Group 1 (the group going up the web and handling the boss in p1) - 1 Tank, 2 Healers and 5-6 Melee (preferably) dps
Group 2 (this group will be on the floor to the North and on adds in p1) - 1 off-tank picking up the Cinderweb Drones, 3 (single target pref) Ranged dps assigned to the Spinners, the rest of the ranged dps need to aoe the cinderweb spiderlings and kill the drones, the remaining healers.
Group 1 on Beth'Tilac in Phase One-
Boss casts meteor burn on you, move out of it.
Boss has a energy bar, when the bar reaches 0 she casts smoldering devastation which will kill anyone who hasn't jumped into a hole in the middle of the web for the cast. You'll have to move back up the web after the devastation.
Melee Dps should dps boss and watch for any small or medium spiders climbing up the web. If any reach the boss, they heal her.
Group 2 on Adds in Phase One-
2-3 ranged assigned to kill spinners who hang by a thread over raid and do minor damage while up.
Cinderweb Drones are large adds which need to be tanked and faced away from the raid. They do a acid cone of damage. They must be killed before their energy bar reaches 0. At 0 energy they climb the web to the boss and heal her.
Cinderweb Spiderlings need to be top priority for dps in this phase, they can be aoe'd. They run to the other spiders, either the drones or the boss, and when they reach another spider, they heal that spider.
After casting smoldering devastation three times, Beth'Tilac will move down the web to the floor and Phase 2 will begin, group 1 should have dps'd her, as you need to begin this phase with the boss between 70-75% hp. If we don't get her to 75% in that phase it's recommended we move more people to the web team (group 1).
Phase two is mostly tank and spank with a tank swap.
We should form an equilateral triangle on the boss. One point on the triangle to the main tank, one point the off tank and one point the rest of the raid.
During Phase 2 the boss will periodically cast widow's kiss on a tank. The off tank taunts and holds her for 20 seconds. At the end of 20 seconds, widow's kiss will drop off and the main tank will taunt back. Widow's kiss does aoe dps to anyone near the kissed target, which is why the tanks stand apart on the triangle. Widow's kiss is also a healing debuff, so the tank with the kiss will not be able to be healed with the kiss active, therefore the tank swap.
The boss gains a frenzy over time, stacking buff for the damage she does to the tank. This is a soft enrage timer for the kill, as at some point it becomes unhealable.
Edit to add L2R video http://www.youtube.com/watch?v=L8OQFgKT ... detailpage