Tbh i dont know if its possible to kill nefarian without getting 1 electrocute during phase 2, u need to have nefarian at about 52% when he lands for phase 3 or else u ll die from adds and ofc the fires will swarm you..
Start trying to get hit by an electrocute in p2 and learn how to heal through it, coz if u manage to reach p3 sooner or later u will realise that is impossible to deal with the adds and the fire for that long.
We have everyone using defensive cdz like, antimagic zone, iceblocks , bubbles etc, and immediatelly drinking a healthstone during the electrocute in p2, u can have dpsers capable of healing using it there, like shadowpriest doing some heal,(elemental shamans maybe , retris etc)
Depending on ur dps u ll have to adjust ur style, we have 2 dpsers in each platform, 1 of em is attacking nefarian only, usually at the end of phase2 we have an add left at like 5% so we finish it off and jump to phase3. There we use a prot warrior with piercing howl kiting the adds away from nefarian with a paladin healer following him, rotating shockwaves/paladin aoe stun(arg,forgot name!) and novas from the frost mage(frost really helps here) to kite them for as long as he can..
Hope i helped.