Ok, so I'm beating bosses at the monster arena, and I've beaten quite a few of them, but I just can't defeat Jumbo Flan. He's immune to physical attacks and he's always on reflect status. I tried using Anima, but she got owned. Is there an easy or at least safe method to defeating this guy? Because I really want those Magic Spheres. Any help is really appreciated.

Speed is your factor to win, jumbo guy is constantly in reflect so you can't slkow him down regularly, I advise using a golden hourglass (or casting reflect on yourself and casting slow on your char).

Now Anima is a good choice but you could do it with your chars as well but here goes anima's way:

Cast haste and reflect on your aeon/and constantly focus until you've maxxed focus, if you need to you cna always shield right before jumbo flan will attack you. Focus is a great factor in this game as it'll level up your magic and your magic defense! When you've focussed nough you should be able tod efgeat mr. flan by doublecasting ultima or flare, pain should also do well. Lancet is extremely effective and can be used to heal anima's HP and MP!

Jumbo flan will cast regen near the end but you can easily dispel that =)