I will add this for anyone struggling with the Trigger Happy method, as for some reason, I couldn't get it to work for me.
In the end I went with 3 Thieves all with Speed Bracers.
A thief attacks twice and quickly. You just wait for all ATB guages to be full and then attack with one girl. After the attack, wait for 1 second (or just under - you will get the feel for it) and then attack with the second girl. Same again with the third girl.
Repeat this formula until you achieve a chain of 99.
It's easier to manage this as all you press is the attack button.
As long as you are below level 90 with all your girls you will be able to achieve a chain of 99 before you kill him. (For the record, I did it with all girls at level 80)