Copy-pasted from YT:
Since I'm slow in evading that monster's jump, I let my twin did the dirty job on this one.
Weak against Slash, Fire and Light.
Charlotte's Ice Needle is a must on this one and it does pretty decent damage when FC (Full Charge) compare to RF (Raging Fire) and RS (Rock Strike). So when the fight begins, cast FCIN (Full Charge Ice Needle) and while the needles aren't launched yet, get close to the beast (not very close or else you'll be hit if he advances) and stick it, once the needles are gone, cast it and do the same thing. When he's about to charge, stand on the platform and wait for him to charge at you then jump. While he's off the screen, move a bit to the right then cast FCIN but hold it down if he's not on the screen yet then when he's in cast it up and quickly backdash while the spell is in effect.
With the big guy down, checkmate!
'Till then, maidoari! *bang*