I  mean,  it  depends  on  your  gear,  I've  seen  teams  with  all  of  those.  I  think  Kael is  the  least  likely  to  succeed.

I am at 21.  Team for 20 was:

60 Queen Eva, Serris, Spider, Astralon. 50 visix

No chance at beating Valks in wave2 on 21 with my team 

I have 50 kaiden (think I need to 6star) to be viable.  Pulled 2 coldhearts last 2x so they are unbuilt.  

I think Visix kit is good, and aura is great.  But I think I may need to 6star as well.  Serris is only sure thing on my team.  But I think I will end up Visix, Serris, Spider, Kaiden, (QE or astralon) all at 60 to do it.  Going to see if I pull anything this 2x wekend before doing anything though

Honestly, any of them. Dark Elves is one of the, if not the easiest  21st floor. The waves are a bigger challenge than the boss itself, but between serris and ovelis, you've effetively shut down their strategy. I'd try it with kael since he's the closest to being useable and if that fails, probably fang cleric since a healer is the one thing your team is lacking.

Astralon? He's Sacred Order, not Dark Elves. Did you mean Astralith? If so, your team is waaaay stronger than mine, lol.

Yes Astralith, lol.  absolutely no synergy with that team.  I know 21 is a manual run I will only have to win once.  Then never again....

... right until the day they add 4 more levels to all the FW series and move Lydia out of reach again. Oracle will then calmly tell us that all is working as intended because she is end-game and was never meant to be easy to get.   :)