So a video appeared yesterday on Warcraft Movies showing a short clip of a Felguard doing a huge amount of damage. Since it's already public, I'll tell how it's done.
Felguard with 4950% Damage Increase in Alterac Valley By oomp - World of Warcraft Movies
After some analysis of the movie, I determined that the buff is retrieved from Magister's Terrace.
1) Clear to Vexallus on normal or heroic mode (heroic mode is a bit faster, but trickier).
2) Create a macro similar to this: /target pure energy /petattack.
3) Make sure your felguard ONLY auto attacks to get maximum benefit (energy spawns are % based).
4) Start attacking boss and whittle him down to 80%. When the first energy(s) spawn, hit your macro until they are all dead. Then set your pet to auto attack the boss while spamming your macro.
5) This is the important part: the stacks disappear AFTER the boss dies, so the key is to take the first 4 waves of adds, then DISMISS your felguard.
6) Either kill the boss or run to the entrance and get back in.
A side note on how to make this easier:
- Take a dps with you that can hold agro on the boss and whittle it down themselves. Set your felguard on the adds and then you can kill all 5 waves. You and the dps then kite the boss to the entrance. On heroic mode, this will net 10 stacks per run out. or 500%. The guy in the movie did this 10 times apparently.
It seems like a lot of work, considering the buff only lasts 30 seconds, but have fun!