Fusion Beasts do not appear in the enemy beastiary.
Ok this time its a little more tricky because now the fusion monster has a higher defense and won't take much damage like last time. So I recommend you get the wind fusion monster and use its special skills because it will cause like 100 points in damge and he will go down in a few turns. Be careful his special skills it may be annoying to deal with
After beating it, it'll unlock the Mad Bull fusion.