first of all, first phase is 1 min. and she has 4,903,346 health, and the first 1~2 second you can't reach full DPS, the last 1~2 second doesn't count either because if she starts air phase then her health will be stuck at 1 until she lands again, meaning you have to endure the air phase.
so you have 56 seconds to down her.
that translates to 87559.75 dps, meaning, you need 88k dps.
an average 80 can pull around 4~6 dps. so if you pug, you need 15~18 average 80 dps players.
if you bring decked out raiders that know what they are doing, 10~11 is enough.
a few more key points,
1, bring more ranged dps and let them
spread out because felmyst can waste 6 seconds of someone's precious time or even kill everyone around him with
this spell. if you are rich, get lots of
this and give one to each melee person one and that way you don't need to worry about Encapsulate.
2, bring a shaman for
heroism.
3, bring a rogue for
expose armor.
4, make sure a warlock keeps
curse of elements on her.
5, heroism 5~10 seconds into the fight, not right in the beginning or in the end.
6, mass dispelling
gas nova is important.
7, tank her in the open region(far from the big tree near where you jump off), and if she does reach air phase, let everyone hide behind the big tree and pray that she doesn't deep breath there.