How do you deal with those boss ninjas? It seems like they're invisible until they've stolen a jewel. For example, I'm trying to get "brilliant" on level 7, but I get to the very last champion and he doesn't become attackable until right before he steals a jewel. I'll admit, it took me a while in the game to realize that the ninjas become invisible as soon as they're first attacked, so I put towers right at the beginning to give myself enough of a chance to get them later.

Anyway, I'm currently up to the penultimate level, so I'll see how I like it. I currently have 7 brilliants, 3 excellents, and 3 goods at level 20.