Fusion Beasts do not appear in the enemy beastiary.
Ok first of all this boss will be hard to deal with its high defense and high HP so with like the medium fusion spirits transform with its opposite fusion either med or high. You may want to pack some Thera Roots as will because high level fusion monsters can take away quite a bit of damage. Just nail him with your special skills and he will go down in a few turns.
Upon its defeat, the Seravi fusion is unlocked.