I think it would be fun to see one of the secret monsters (nessa, mabu, or tama), though nessa would probably be bad, but Collapse is OP so who knows (if you do nessa it does really good against targoth)
XD Well I think is imposible or just really hard. I got the legendary ominous bust (or something called like this) in the first area. I bought a lot of potions and nothing XD I cant defeat the first area.
Ominous Idol is much stronger for large teams because it affects everyone. With a single monster you are probably making the enemies get more of a benefit than for yourself.