The boss is vulnerable to Air Magic, therefore yellow spells are the weapons of choice. However, since he uses Earth Magic to surround you with Stalagmites, your Lightning has reduced effect (too many targets - you will hit Silarda if you target him but not all his summoned creatures). It might be a better tactic to combine Sentinel (for soaking up damage and some AoE) with Static Field for some focused AoE. I used a Chakram to clean up the battlefield and switched to a melee weapon once the mage closed in.