The problem is at that point in the game, Tellah's level 3 spells are your most damaging form of attacks, but he can only cast them 3 times before he runs out of MP. If 2/3 of your magic is being waste on the arms, that means only 1 fire 3, bolt 3 etc is going to actually hit the body. It would probably be worth your while killing off both arms first with physical attacks, then reflecting magic at him before they grow back.