This is majorly cheesy, but use a weapon with the Freeze property on the bosses, which makes them immobile and unable to cast or turn invisible. Then either switch weapons and hit them with your most damaging weapon or continue using the weapon with the Freeze property.
Also, consider investing at least 3 or 4 skillpoints (if you have any left over) in Hell Spikes. That spell will damage invisible enemies if they are caught within the area of the damage. This can come in handy against bosses that are too wussy to face you head on. <img src="/ubbthreads/images/graemlins/tongue.gif" alt="" />
