Here's how I always beat him (with Celestial Weapons equipped, of course):

1. Hastega
2. Reflect on one character
3. Slow on Reflected character- will reflect on Jumbo Flan; can't cast directly on Jumbo Flan 'cause it has Auto-Reflect
4. Doublecast Ultima
5. Copycat
6. Repeat 5 as many times as needed

It works for me every time.