I am not sure what battle this is based on your post. I think you are talking about the battle where your magick is disabled. If this is the right one here are some quick tips.
Set your highest priority gambit to Ally any: Phoenix Down. You will want to have all of the Phoenix Down augments learned. If you do and also equip the item that improves item potency, you will revive allies with full HP.
A good gambit with for X-Potion is a good idea too. With all of the augments for potions unlocked they will heal for 3600 pts. This should be almost a full heal for your characters.
One of the most important things is to keep the Dark Lords at bay, so that they do not overwhelm you. I don't remember exactly how high their life total is, but if you set a gambit for Enemy HP < 10,000: Attack higher than your main attack gambit, your characters should focus only on the Dark Lords when they are around.
That is pretty much all I did to beat this Esper. I think my level was around 40. If you do not have the Item augments unlocked you may have a very hard time with this fight. I would suggest making sure you have them before taking this one on.
If this was the wrong battle, give me a better description of where you are and I'll figure out what boss it is.