I have an idea!! (Again)
Why not implement a status that Colossus only has? To be original about it, let's call the status "Colossus."
Which means that Colossus is immune to Poison, Gravity Pull (Because it's so big, it has it's own gravitational pull to counteract..), and Freezing/Congeal?
If you can somehow fit that in the text, I think it would balance it out from making Mono too powerful. Although, Ground Smashing sounds awesome..
All in all, I'll take 6 of them.