3 things I need to know about Arans

1) I was under the impression that Final Blow, like Combo Smash and Combo Fenrir, does not get a damage reduction against mobs. Assuming that's true, without SE (or at a low combo count), Final Blow should be worth using if it can hit 2 or more monsters. At a high combo count and with SE, you'd probably want to hit 3.

2) As Orpheus said, the Crit Ring is better for any class that has any amount of crit, including being buffed with SE.

Werent the rings only usable with adventurers?

There's currently no reason to think that.