Man-Thing can regrow himself under his own power, ie, doesn't really even need nearby vegitation/swamp to regenerate. Colossus has no way of actually putting him down (Hulk can't do it).
On the flipside, Man-Thing probably couldn't do to much to Colossus, unless Colossus felt fear, man-thing's burning touch has done serious damage to some tough people.
The fight probably goes like this:
Its a draw any time it doesn't frighten Colossus, a win for Manny any time it does.
