How to kill Baal; mage on impossible

[jake21]

Ok. I'm playing mage on impossible and Baal does not want to die. Current troops are paladin, blackknight, knights, royal, horseman. I attempt to move the blackknight to the front so they can attack baal as often as possible on the right side (sword is on left side). Once I get baal about 1/2 dead he begins to summon new troops every other round and i can barely touch him.I'm thinking I should keep the knights but change other troups into some sort of range attack but not sure what would work effective here. For spells I cast stoneskin on the black knight only and then attemp to use damage spells on baal (mostly poison skull or pole axe; but occasionally a death star.