Despite what you may be inclined to think after looking at Aqua Spheres, they are not prone to being damaged by magic spells. Physical attacks are the way to go when taking them down. As far as their offense goes, they can hit you with some water damage, but will sometimes waste their turn casting Fast Cast on themselves. What that does is increase their casting speed. The problem is, they don't have enough HP to waste their turn casting that and still get away with casting a spell on their next turn unless you completely ignore them. Don't do that.