Not sure if this will help, but for me I find going heavy offense and watching for their heavy attacks to jump or block (to take less damage) seems to work best. Something not mentioned in this video is to trade out you characters to help them regenerate.
As far as being done with this game. Did you download "Heroes and Heralds"? I don't think it came on the card, I had to download it to unlock it. Or maybe it was in a patch. Anyway, as far as Single Player goes on this game, I feel this is the best mode as it gives you the ability to figure out what cards to use against the other team, and you don't always fight 3 man teams and what not. Best mode in my opinion.
Here's a video, in case you haven't seen it.
I almost didn't want to believe this.
Attack when he's not attacking. String attacks together to do COMBOS. If you do enough damage, he will stagger, at which point keep on doing damage. Example: when he's charging up for his big beam super attacks, you can prevent it from happening by using lots of big flashy screen-filling supers of your own! Yay!
I'm positive the reason you can't get past this supremely easy last boss is because you refuse to stop pressing buttons. Block when you see an attack coming, for God's sake. BLOCK and PUNISH.
Also, don't use Phoenix (Jean Grey).