Let's start with a roster screenshot and potentially screenshots of gearing on some of the champs you've been trying to use to do these, and go from there.

The tl;dr for Fire Knight is we need to bring the shield down and often keep the turn meter down  (or have a relatively tanky/heal heavy/revive heavy team), otherwise we're in trouble due to the self healing and damage FK will continue to put out. 

My first impression:

Your champions all need more speed....

Drop Dark Athel, build and insert Ronda

Replace Deliana with a build Visix

Drop UdK, build and replace it with Rhazin...

You first want to have multi-hitters in your team and champs who can reduce turn meter and/or place heal reduction

Champs like Coldheart or Alure would be supreme in the normal levels of FK.

As an example, I just threw in the champs on level 18...some are not fully equipped or fully mastered...none has their soul activated...but all are at around 170-200 speed

You have the champions...you need some gear..;-)

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As noted above, your key in normal FK is to use multi-hit champions to bring down his shield as quicly as possible. Honestly if you just throw in Rhonda I think you will be fine for level 10. I don't even think you have to 6 star her at this point. Disable her AOE or if you are fighting him manually don't use it. The A1 will work better. 

Given what we're seeing now, it might be even more helpful to see the rest of your roster, in panoramic form. One or more of those turn meter control champs would go a long way. :)

I struggled with Fire Knight until someone suggested Alure.

Not only does she have a 3 hit A1 which helps drop the shield she can then help keep the shield down with her 25% reduce turn meter on critical hits (also on the A1).

If you have her she is great for FK. She needs high C.RATE, ACC & SPD.

try Cold heart ,Ronda, Apthocary and Athel  and Kael ,, all OVER 200 speed

from a Free to Play player ............

This is my succesful setup to pass the fire knight level 10

to be succesful, i do need to deactivate the auto when facing the fireknight.

Ruella is an asset since she does a triple attack, and once the shield is open, she can use the no respite 

and loki his bandit flash helps to increase damage.

hope this is helpful for other players struggling with this level

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i managed to pass with this setup to pass level 10

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Spam post a few up, mods.