Are shields OP? I find that I'm almost forced to use them or my characters are dying all the time. I mean, the shields almost double both magic and physical armor. And there's very little penalty for using them because of bouncing shield.

An INT build with some points in Necromancy and Warfare for Mosquito Swarm and Bouncing Shield can be extremely powerful; absent those skills, they die and don't really get the job done.