Before fighting this boss, make sure you have a good ranged weapon and a good melee weapon, each mapped to one of the attack buttons.
This boss is very easy. He has four heads protected with armored flaps arranged around his perimeter and a vulnerable belly. When the battle starts, run up to the first head and attack it from underneath it, standing slightly to one side and using a melee weapon like a sword. From this point you'll miss some of your attacks but you won't get hit when the heads spew their green vomit attack. Attack the head until you've taken off half of its hit points and the armored plates fall off. At this point you won't be able to do any more damage to it. Run around the boss to the next head and repeat until you've done all four heads. Once all four heads have had their armor plates destroyed you can finish off the heads. Using the same technique (melee weapon, slightly to the side) attack the heads until they have no hit points left and are destroyed.
While you are working on the heads, the boss will occasionally fly up in the sky for a while before slamming himself back down on the ground. When this happens, switch to your ranged weapon and use LT to go into zoom mode. Aim up and fire away at his vulnerable belly. Back up when he comes crashing down to avoid getting smashed. While the boss is on the ground you cannot attack his belly.
You'll need to destroy all four heads and the main body to beat the boss. Since it's so easy to avoid the boss' attacks this fight should not give you too much trouble.