Hahahaha... i used to have this problem like u, so i think a little help should be okay.
First of all... what level are you now? I suggest that you fight this Esper at level 50 or above that. Have the leader equipt with small range weapon (your best 1H or 2H sword...or an axe, katana... whatever that is to your convinience) and also get him a bubble belt to double his HP. For this character, first gambit should be ally=any, phoenix down or rise; second gambit is foe=strongest, attack. (third gambit and so-on is up to you).
For the second character... equipment is same like the leader. Set first gambit to be ally=HP<60% x potion or cura (curaja would be better if you have it). Second gambit to be foe=weakest, attack (to get that annoying slime). Third gambit is foe=party leader target, attack.
Third character is best with long ranged weapon (such as bow or gun), equipt also with black belt (so at least there still one character remain to immune when Cuchulainn cast Disable). First gambit is ally=any, remedy or esuna. Second gambit is foe=party leader target, attack.
Notes that before entering this fight, beter have level 3 remedy so that your remedy can cure multiple bad status such as SAP, disable, poison... etc. Also... cast all of your supporting magics such as shell, protect, and regen. try to re-casting them when the effect wears off during battle.