Fusion beasts do not appear in the enemy beastiary.
Ok this time its alittle 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 light 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
Upon its defeat, the Charon fusion is unlocked.