the fastest way curret patch I know off.

Ultraboost up to the checkpoint where the centry spawns, checkpoint and rocket ride backwards around the earthmover, snipe the electricity box on the crane with the rail cannon to open up the secret path inside. Once inside, go break the two idols on the highest wide platform, and as soon as you break the seccond idol restart from checkpoint. go outside and find a pit to fall in (i only really use the pit at the very botom of the earthmovers feet, but there might be one elsewhere on it thats faster to ub too) after falling the game will place you, not at the checkpoint, but back in the idol room, and after breaking the idols and leaving out the top, the earthmovers head is unloaded, so rocket ride/ub to the brain and it will start lobotomizing itself once more, then its just getting to the end to stop the timer.