I noticed that each time I am fighting against a Defender Hero, the AI is doing a VERY poor job at using it and the Hero spend the whole fight casting guard on itself.

Obviously there's something wrong with the AI programming here; casting Guard may have some sense in very specific situation were the hero is recievieng the attacks while other are damaging the opponents. But it was not the case in all the fights I had. The AI hero was just like a maniac in his corner of the map casting guard on itself from the very first round to the last strike that killed him.