This was really fun! I liked the variety of characters, they all played fairly different. You should definitely make a sequel
I liked the graphic effects like when you got the piece of power, or when you went berserk. It was a very nice touch.
Some suggestions:
Like the others said, it
does get a bit repetitive. I don't think that introducing randomness is the solution here. Instead, I think just increasing damage in general is the way to go. Make the player, and the enemies, die faster. Maybe by a factor of about 1.3? Also, maybe decreasing the length of each stage, or maybe giving you 3 lives before it kicks you back to the beginning or something (To counterbalance mistakes being more costly). After all, each level is only really hard while all the enemies are on the screen. Once you kill one or two, the level is a lot easier, and more boring.
I really, really disliked the berserker. It was never really worth it to go berserk unless you knew you were going to die in one hit anyway. Oh - maybe if you gave him increased speed, too? So that he was actually better at dodging. I'd have to try it, but I think I could see myself actually using that power.
I think a stabbing character would be fun. Or maybe one with a hammer? You had a lot of interesting characters, but I think more variation in the basic attacks would be fun, too.
The secret character was way overpowered. I know that's kinda the point, but I think it would be fun if you toned him down a bit. I thought it was a really interesting character concept, and I was sad I couldn't actually play with him in a challenging environment.