I just recently completed a run of the bosses in Megaman Legends 2. Here were the conditions:
-No Damage
-Very Hard Mode
-Customized BGM 8)
IMO Rush Mamoo was the hardest one to go against, as taking on that monster without at an offensive weapon is a nightmare. Yes, he's even worse than Sera. Enjoy!
Men legends 2 had some cool bosses.
And I forgot so many of them.