Once the battle against Colossus begins, run away from it. You will likely notice that there are gates that will only open when Colossus is within a certain distance of them, and may further notice that in front of each gate is some dynamite. This is because only dynamite can hurt Colossus, thus you need plenty of TNT in case you have a garbage-tier throwing arm. Simply pick these up.

That's basically all there is to it. There doesn't [at the time of writing this] seem to be a limit for how much dynamite you can carry, so you can just keep running away and snagging those pickups, and it won't be long before you've got enough to blast your way through what's left of the game.

Honestly, it's kind of pointless to do this, since there aren't -that- many enemies between Colossus and the last boss, and the last boss is kinda hard to kill with dynamite. Still, just thought I'd put this here because there aren't any guides for Dementium II HD as of now, and I wanted to put something up.

So... Have fun making things go boom!