You first are introduced to Clank's gigantic robot form in this game on planet Quartu. After following the left path for awhile, you will reach a little man by a machine on the ground. When Clank steps on the machine, he is transformed into Giant Clank. You can punch things with your fists, shoot out little homing missiles, jump around, and shoot a special energy blast attack. As Giant Clank, make your way to a circular arena entrance, blow up the door and enter, where you have a fight with a rather large robot called the Ultra-Mech and some flying helicopters. Simply blast away the helicopters with your missiles, and attack the Ultra-Mech with your missiles and special, and punch him when possible.
You next use Giant Clank on the final planet, Veldin, to take out Drek in the first of two final boss stages. After you battle your way though the level, near the end you are able to transform into Giant Clank. Do so, and destroy everything in your path until you reach the large circular area, where you meet up with Drek and first fight him. Attack him until he runs away, and from there its Ratchet's turn to take him on.