They're all very solid - for faction wars.

Outside of FW the only one of those I've used is Kaiden, and in extremely limited fashion. But that's largely dependent on your roster - who else are you running? If you have Arbiter, for example, Melga is almost entirely useless.

Outside of FW, trying to figure where i would use them.

Thinking reinbeast and Kaiden would be good for CB but can't think of anything else for them. I don't have Arbiter just yet but working on that now.

right now 

AD- Miscreated monster, godseeker, Lysandra, ninja

AO-Ninja, lysandra, bad el, seeker

CB- Ninja, god seeker, runekeeper, MM, lysandra

DT- God seeker, lysandra, MM, Ninja, bad el

Kaiden isn't really worth running for CB. He only brings ATK down, and lots of other champs do that. Reinbeast doesn't even bring that. He's definitely not worth it for CB.

Your CB team has its own share of problems, and I'd suggest maybe starting another thread for that. But for right here - no, I would not bother with any of those four champs. The ones you have are all easily better.

I  don't have  Cornelia  and  I  am  not  familiar with  her  skill  kit.

The  other  3  heroes  you  mention  -  I  have  used  in  my  Faction  War  teams.

The  Dark  Elf  only  had  2  revivers  when  I  beat  Stage  21.

Kaiden  or  Blind  Seer

I  used  Kaiden  because I  couldn't get  my  hands  on  Blind  Seer.

Kaiden  help  me  3  star  a  few  Stages  leading  up  to  Stage  21.

I  didn't use  Kaiden  on  Stage  21  though.

The  Skinwalkers didn't have  revive  hero  when  I  beat  Stage  21.

Reinbeast  was  a  critical hero  because he  offered  A1  Healing  +  Shield.

The  Dwarf  Faction  only  had  2  revivers  when  I  beat  Stage  21.

Maulie  or  Melga

You  really  need  a  Reviver  for  them  because  the  Boss  hits  super  hard.

He  will  usually  kill  your  champs.