I watched it one time when i was little (90's) and every year or so i try to find this darn movie but have always failed:

its set on a future earth that is controlled by an alien race that took us over. One badass human dude had a giant ass transformer-like robot that he fights the alien robot with in a 1 on 1 battle for earth.

Noteable events in the movie:

in the beginning, the main guy jumps off a giant waterfall in a jungle setting to escape the aliens, who then think he is dead.

in the future, some humans and some human chick find him and recruit him to find his old robot and challenge the aliens to an epic 1v1 that will decide who owns the earth.

one of the humans is secretly working for the aliens and sabotaging the human robot.

The chick (main guys now love interest), who i think was also a badass mechanic, gets into the arm of the main dudes robot during the fight to like.. stop the sabotage hack from ruining the robot.

Humans win.