Get a 100% game completion rank to unlock a bonus FMV sequence.
Defeating Cortex
Avoid the shots and bombs of Cortex. After that, he will fire big green balls at you. When they reach you, throw them back by spinning. Do this three times.
Beating cortex and your robot brother
When you come up against cortex fo the first time wait till he has fired his bullets at you and watch out for the aiming sign and then when he starts to throw bombs at you, again watch out for his aiming sign. Then he should use a plasma bomb when it is about to hit you press X. Note: continue to do this until his health is out. Then you V.S you robot brother just dodge all his attacks until he shoots out a plasma bomb and when it is near you press X and bounce it back, do this 2 times and then he wont use his chainsaw, do this another 2 times to blow off his rocket launcher, then he will only shoot out plasma bombs, deflect the bullets back at him 2 or 3 more times to beat him and finish the boss part.