Ah... I didn't get any time to play yesterday, but now I'm finally there.

And yes, that boss is by far the toughest battle I've encountered so far. I'm currently at level 18-19 (except Chaos who just went to level 20). I've only fought the boss once so far... and in the begining it didn't seem too bad, actually... but then, they started to use those ice storm and instant-KO attacks, and I went down pretty fast. I managed to kill one of them, but then the other one seemed to get about twice as fast and ONLY use the most powerful moves along with a new move called "Thunderstorm" which did about 400-500 dammage on all the characters. And most of my characters have 500-600 as max HP at the moment so... yeah.

It's NOT impossible, though. And I WILL beat them the next time!