I defeated Siemon (Albedo's AGWS) on my second try and the blue-masked guy on my first. I'm now very close to the end of this challange, and I'm currently at level 26-27.

Siemon was actually pretty tough. Its HP took some serious time to get rid of, and all its moves could kill a non-defending character in 2-3 hits... which become a bit of a pain when it boosted and used moves that hit all the characters. Mr. Blue-masked guy's first form was suprisingly easy and I had no trouble with it at all. It's second form was a bit more difficult, but nothing all that bad.

The final boss will probably be a hard nut to crack, though. "Black Omen" can do some major dammage to leveled characters on a normal play-trough. In this case, you'll probably need to defend in order to stand a chanse to survive it at all.