Cast Thundara. One hit can do about 2700 damage. I have all of my character learning all of the magic and it's easy to beat him with Thundaras. Bansat will probably buff you with Protect, Shell and Haste, make full use of that!!!! Use Cura often too. If I'm right, he's not immune to Slow and Immobilize, the walkthrough is right, you just need to cast it often, don't worry, I always have to cast the same magicks more than 20 times for it to work, so you're lucky. If you think Ice Shield is not helping, unequip it, and you'll see the ice damage you get doubles.