Have Colossus #3 use a long-range attack (just run away from him until he does this). Quickly run to the top of the sword and hold onto its right edge. From the sword quickly jump on his armor on his legs and from there, once he lowers his arm a bit, jump back to the arm above the wrist. You have to do all of this in one constant motion. If you wait on the leg armor for too long, the Colossus will just keep walking around and not lower his arm anymore (in this case drop down and let him do another long-range attack to retry).
Continue with destroying his vitals as usual for the trophy. Be sure, however, for the boss to not hit the metal ring in the middle of the arena or his wrist will break (thus voiding the trophy).
Check the video:
All credits to PowerPyx