Fusion Beasts do not appear in the enemy bestiary.
Ok first of all this boss will be hard to deal with, it has high defense and high HP, so transform into it's opposite fusion, either medium or high level. You may want to pack some Thera Roots as well 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 Forron fusion is unlocked.