How do you defeat Metal Gear Rex

Throw a Chaff grenade first to keep Rex from moving.

Then attack the RADOME on his shoulder with the Nikita Missles.

Gray Fox will help you out after that.
Then keep doing the same thing.

Chaff, then shoot for Rex's mouth.

I have this strategy where you use a Chaff, run BEHIND Rex, and attack the mouth underneath him or so... Good luck.