This will be a tough one. My friend wants to find this game he played while he was young. This is all he can remember:
It was 4 players and co-op. It's a completely 2D game. The characters were megaman type guys all with different colors. He's unsure if they had different abilities or were simply different colors. It was not side scrolling, you climbed upward to progress. It's mostly about robots as all the enemies are robots.
Apparently, some of the platforms you could walk around on upside down.
There were bosses every 10 levels or so and there were tons of levels.