Cubix Robots for Everyone (2001)

Connor, Cubix, and the Botties (in Dr. K's battle bots) attempt to save Bubble Town from Dr. K's enormous robot. Eventually, they wind up having to save each other- as Abby gets sucked back inside and Connor runs to her rescue. Eventually, it's comes down to Cubix and the big monster and Cubix's only chance to win is by transformation. He changes from mode to mode until he overheats and overwhelms his opponent. Unfortunately, he overheats and overwhelms himself, too and Connor is the only one that can revive him.