Colossus heavily relies on brute force when facing his enemies. He doesn't name any of the techniques he uses but only lets out a warcry, a roar or something similar everytime he is about to perform one. All in all, all the techniques he so far possesses are two;
Shockwave
Using his mace's metal head, he can cause a shockwave by plunging deep in the surface and swinging it upwards, he can cause a shockwave which can knock back any of his enemies who lie in the direction he causes that shockwave.
Iron Stomp
Colossus can easily slam any physical surface with his mace, denting it or sundering it. This can make the enemy loose his balance or even use the remove chunks or parts as weapon such as blowing them with his mace and sending them towards his enemy.
Two Handed Blow
His attacks while wielding his mace with two hands can be quite lethal indeed. Let's just say that while he was younger, he could tear apart a whole pirate ship in two. It's best to avoid them cause getting hit while he wields his mace with both hands could really make you see stars circling around your head.