I Wanna Beat the Random Numbers is pretty nice, featuring four short bosses of varying length. A nice thing about this one is that it gives you a health bar, so definitely good for practice! A bit on the short side, though.
I Wanna Be The Roman also features a health bar, but a far lengthier fight. Perhaps give it a shot after Random Numbers.
Personal fave is the avoidance fight in Rose Gear. There are two versions, but the advanced one is only available by collecting all four hidden items ingame (which is hard). The normal version is decent length, manageable RNG, and nice to listen to.