How do you beat devastator in transformers revenge of the fallen the game

To defeat Devastator you must destroy all of the weapons that he has, firstly he has rocket launchers on he back, you must attack these from behind (stand on a building and wait for him to pass), the weapons are tagged with red markers, then guns on is back legs, again attack from behind, then guns on front legs, attack from either the front or behind and then finally the head, you need to attack a power supply on his forehead