LB is the best scaling offensive spell ingame without a larger cooldown, because of the -1s casting time from lightning mastery.
Conando has excellent gear (i guess 1500+ dmg buffed in this fight?) and a shadow priest, so it's near to "infinite" mana with his 47% critrate. But those stats are exeptional, i guess only reachable with
Staff of Disintegration (can't check armory from work station).