Ok, no need to worry, there is a simple way, you'll need
Cat Nip
Haste bangle x3 or (as I had) 2 Haste bangles and a speed bracer.(basically 3 accssories that have haste equipped to them)
Highroad winds GG(for the First Strike ability)
all 3 characters as gunners and Trigger Happy Lvl 3

Now,
equip one of the gunners with cat nip and get them on critical health(Yellow)
Go into the battle and use your first gunner to use trigger happy (with haste on it'll last longer)
DONT USE THE OTHER GUNNERS TRIGGER HAPPY YET!
as soon as the first gunners done quickly get the second gunner to use trigger happy, then do the same with the 3rd. By this time your first gunner should have her turn again.
keep doing this and Chac will be unable to attack.