Two methods, the cheap method and the not-cheap method:

Cheap method: As soon as the area starts to get dark (a sure sign that Mericarol is going to appear soon), hightail it for the entrance. Now, tiptoe back into the room as slowly as you can. As soon as the targetting reticles focus on Mericarol (the Merillias should all remain rooted, if one becomes unrooted, finish it off with Zonde first), start blasting away with Zonde. You should be just out of reach of Mericarol's AI so it will just sit there and do nothing as you blast it to smithereens. (Note: Depending on how high your Zonde is, you might not have enough range. Mine is pretty high, but I figure it should work with any Zonde around 20+... if you don't have Zonde lvl 20 you'll probably have a lot of difficulty with this area in general, as both Merillias and Ul Gibbons are weak to Zonde on Ultimate).

Not Cheap Method: Run around dodging that fireball Mericarol shoots (instant death to me with Deband with over 550 HP) and taking pot shots with Zonde at either Mericarol or one of the Merillias that's bound to pop up.

Gibbles has two basic attack patterns to watch out for: Smashing your face in and jumping on you and smashing your face in. He's weak to Barta, so hotkey Rabarta/Gibarta to freeze him and your most damaging (usually Barta) ice spell to damage him. Make sure to cast Deband--you'll be taking hits--and Jellen on him, if possible.

Now, try to freeze him with Rabarta/Gibarta (if you do enough damage, you can also stun him with this), if he knocks you down (which would be any time he hits you), immediately cast Resta then your freezing spell. If you manage to freeze him, cast your damaging ice spell as much as possible. As you see that he is ready to jump you need to run towards him and then underneath him as he jumps and he should miss you.

Alternatively, you could try running in circles and casting Rabarta... but that's probably an exercise in frustration. You should be able to ignore his Gibbon buddies... they'll either be out of range or get killed off as you spam Rabarta to freeze the boss. If there happens to be one in range to cast spells at you but too far to hit with Rabarta, try to take it out asap, because Shock from Gizonde can be deadly (and Megid is worse).

He's weak against Ice like the little Gees that are with him. So just run in and cast Rabarta as much as you can, healing when you get hit. Every now and then, he'll fly to the other side of the room and shoot you with something, but by the time you get up and heal, he should be within or close to Rabarta range.

The advantage of casting Rabarta instead of targetting Gi Gue (if you can manage it) is that Rabarta also kills the Gees that are with him and the poison mound thingies that pop up out of the ground.