Felmyst as most of you know have this nasty spell Gas Nova
http://www.wowhead.com/?spell=45855A blast of fel-infused gas eminates from the caster, dealing 1885 to 2115 Nature damage and draining 3000 health and 1000 mana every 2 sec.
Cast time: 1 second
Cooldown: 5 seconds
Radius: 100 yards
First thinking how to do pull.
Similar like archimonde.
Paladin go foward with 2-3 rogues, once he come close Felmyst he bubble and rogues start to attack Felmyst with appy poison who increase time between casts by 60%.
Means aura cast time will go to 1,6 second cast, verry fast but i should be possible to interupt spell cast just like Twins spell casting.
This is unconfirmed, just a theory.
If rogues get Gas Nova on them they will survive one mistake with Cloack of Shadows, like it brake stealth
http://www.wowhead.com/?spell=31224Boss enrage after 10 minutes at least on official.
Means we need to have full epic geared raid, first to kill Brutallus then Felmyst, allso raid musnt panic and healers need to stay with cold mind.
This is dps race like brutallus just this time it depends about how fast healers will stay without mana.
About resistance gear and potions.
Warriors and Paladins can make resistance gear for them self but that isnt MUST, even so it can work if some paladin clean warrior as duo but we need to force ppl to create this and in last cases it fail.
Other solution could be this potion
http://www.wowhead.com/?item=22844#created-byfrom my way of thinking i belive this one is buged on this way.
It absorb 4000 damage on hit. If damage is lower than 4000 it wont be consumed, if damage is above 4000 it will consumed.
Since Gas Nova deal 1885 to 2115 Nature damage it should absorb all damage, need someone to test this, it should be same with all other kind of resistance.
Another thing what could be good will be
http://www.wowhead.com/?item=24095lets say all 5 people have this neck in one raid group
1st use neck, it got consumed by damage then after him
2nd use neck, it got consumed by damage then after him
3rd use neck, it got consumed by damage then after him
4th use neck, it got consumed by damage then after him
5th use neck, it got consumed by damage
it will consume lot damage but it will work like this only one time since it have 1 hour cooldown.