RUBY WEAPON - this well-known setup practically guarantees victory:

EQUIPMENT
Ragnarok
Mystile
Ribbon

WEAPON SLOTS
Knights of Round=HP Absorb, Master Summon=MP Turbo, Mime=Command Counter

ARMOR SLOTS
Fire=Elemental, W-Summon, HP Plus, MP Plus, (anything else)

Cast Knights of Round x 2 once and the rest will go on by itself.
Cloud automatically give priority to the tentacles, too, so don´t worry about that part.