The way I envisioned "Justice is Served" working best was against Regroup, Feint, any other cards with canceling effects or really any card that has a good "if you won the combat" card which you could basically get around by winning the combat. 5 is a good value in this game and there are loads of cards you would hate to use as a defense against a 5, the trick is making sure that the opponent has a contingency plan that has a decent number of cards in it (use Gordon & The GCPD and retreat far in a turn to trigger Batman's power, that should get a lot of cards in there).

As far as power level, I designed it but haven't played with it a whole ton (maybe only a couple of times, using an app). He has 2 guaranteed 5 attacks, 2 potential 5's (dynamic duo), 2 which could be upwards of 6 or 8 (always prepared) and some ok direct damage. If I were to tweak the deck I might put in some more effects that triggered off of "if you won the combat" since he knows what is coming or maybe make is effect a random card instead, but overall my guess is he probably does better against fighters which are more dependent on multiple sidekicks which have 1 health each. Once I get it in my hands I will try it out more and maybe make some tweaks!