We have pretty usual Karazhan group, thou @ Aran we just swap tanking warrior for warlock that's on standby.
We are not trying to prevent him from using mana but contrary - we force him to use mana. We let him to use firebal, frostbolt and ofc all his specials, being careful for avoiding it. We interrupt only his AM cuz he already spent his mana. When we get him to 45-50% he's almost @ lost all his mana. He going to drink, healing all of us on 8K+ HP, mass pyro during that time everyone have ready Hpot + Hstone ready we pop them up and continue fight to 40%, @ that time 2 warlocks start to banish + fear and others just burn him down. Yes - we intend to change our strategy when our gear become better and when we do him many times and become more confident, but same as Ragnaros - how many guilds killed him b4 the sons on the 1st tries?