You cannot harm Rubicante while he covers himself with his robe

Either you must damage him with physically or set your defensive

When he opens up use Rydia's Shiva and use Edge's Flood
Using Blizzard spells makes Rubicante counter with his own Blizzara on himself causing to hurt himself instead! (Only if he opens up his robe he heals once he covers himself with his robe)

Good luck