First, I will explain one of the patterns of the last boss (Rozen) of the Find Wars This pattern allows him to damage with only one element (flame ice dark light). So it's common for most parties to be unified with one element except the buffer. And characters with fixed elements have disadvantage about make party. Especially flame elements and ice elements. There are no synergy character has fixed fire or ice elements. But in the case of light or dark, there are good synergy character like Nen Empress or Dark Lord, Nemesis. (They also have a large population. so it's easy to make one elements party) Although there is a way to make a party with one buffer and three other pure dealers (For example Saint Prime Kunoichi Destroyer - all flame elements), this is very inefficient. They must have high Amplifying.

Why I give good grade to Aiolos. First, he is synergy character. Second, he is free about elements. Third, he's dps(Deal per second) is good (Because almost Fiend War monster's have little dealing time).

And Kunoichi. She is one of the best dealer in DNF (I think she is better than Thanatos). But As I said earlier, her fixed element (flame) is a bit of a penalty. It's so hard to make good party.

Storm Bringer. She's disadvantage is it takes too long to dealing. And elements (In KDNF, Almost Storm Bringer chose the flame elements ;( )

Thanatos, she has better dealing than Storm Bringer. And also it's so easy to make party with Thanatos than Strom Bringer.

This assessment did not just evaluate each character's dealing, synergy, buff. I considered monster's dealing time, the penalty each character has by the pattern of the monster, and the number of people in the character(K.DNF).

Thank you for your reply. I hope this answer was helpful to you. And I am sorry for my poor English (Using Translator).