Just out of curiosity I play the same team offense and defense in PvP.
Offense I have not lost one fight yet. It was very close against Nicko's team and he had the first strike. I was basically lucky that my Bomber Boy froze 2 of his toons twice in a row (perfect attacks). Otherwise I really would have lost.
But that also shows how much better a human player is.
It was also mentioned a few times here that the point system may need to be changed. Firstly it really is kind of frustrating loosing so many points because you need to sleep at some point. Secondly the AI for the defense is obviously no match at all for a human player.
I also don't think the AI is that much better than in Campaign mode. The toons in Arena hit just much stronger (skillshards) and you face a very powerful boss each time. Plus you are facing toons that all equipped with extra skills (freeze talismans and so on) that you don't have to deal with in the campaign.
I really would like to see some sort of compensation for the Defense as this is out of the player's control.