Shield toss is based on your armor, like reactive armor. So your mage will have to put points in strength to wear armor that will allow the shield toss to be useful. The enemy spending their entire turn moving isn't the bad thing you seem to think it is. By the time group advance to get to my dps, half of them are dead, and then they are all nice and surrounding my tank after they move, for him to nearly finish them with reactive armor.
Tank builds in this game are not classic versions of tanks. Picking up a shield doesn't magically turn you into a damage sink. It just changes what attacks you use. Polymorph also gives you tentacle whip, which has nothing to do with your weapon.
