Check this Out, Its really quite simple:

The best place to get away is on one of the higher up ledges on the sidewalls. One of the most deadly attacks is his lunge, he holds onto the floor and kicks you, that’s painful. When he uses Poison Breath it clouds a lot of area so there is no way of getting away from it. It will slowly take away some Hp for a little while. Not only will his breath do this, but the green stomach acid around the room will also do the same. So don’t stand on it. When he falls back the mouth on his front will open up, revealing a dark core. Lock-On to it and start slapping it around. It will drop some HP Balls every so often. The only other tips I ca add are to stay away from the acid and keep your HP high.

What I did was stay as far away from him as possible and use the Fire spell. then attack its core. When it starts to attack again go back to where you where and use an Ether and use Fire spells again. Keep doin this and you'll eventually beat him.

You had trobule w/ this ghy? Ha noob! no just joking! I had trobule too, okay, my tip is to train your guard, when he uses his hands to attack w/ his body, guard it then attack him, after this he may open his mouth and the dark portal will appear giving you life balls, don't attack near their it gives you less damage on him.... and about the time when he sprays green stuff, take it like a man! then, cure....

If you managed to survive your first encounter with the Parasite Cage, the next fight shouldn't have proven too difficult. It's essentially the same battle save for the monster's increased HP, poison breath attack and the addition of the damaging acid that coats the floor.

• Cast Aero on yourself.

• Jump off a platform and attack the monster with aerial combos (attack from the front as the monster cannot be damaged from the rear).

• Once you land, jump off the ground immediately and retreat to a platform. Attack in the same fashion.

• While fire is effective against this monster, I recommend using magic solely for casting Aero and Cure. Use these spells when necessary.

• When the monster sways backwards, it's stomach will open. Attack the inside of its belly, making sure to pick up any HP/MP balls that may be dropped.

That's how I go about the battle.