Mana Siphon + Life Regen on a Rare boss ?

Perhaps there should be a list of rare special attributes that shouldn't conflict with eachother, i'm a sorceress and despite using my whole array of spells, this boss appears to be hardly killable. He runs faster than me, regens his life while i try my hardest to put distance between us, steals my mana in his 10 meters wide circle self aoe, deals passive lightning damage in that very same circle, how am i meant to kill that guy as a Sorceress with no minions ?

Mana siphon AoE only siphons on the outer ring. You are supposed to stay in melee range inside the ring to stop the siphon. Its a gimmick to force you into melee range.