not sure what level you are looking at, but the key is multi-hit/Ally Attack/Counterattack to take down his shield and then TM control to prevent it from going back up.  Looking at your roster, I'd suggest High khatun lead, Skull crusher, Lordly legionary, Kael, and Armiger.

Who are you currently using in your Fire Knight squad?

Fellhound and Skullcrusher are great potential includes in the team, as Skullcrusher Counterattack can really do a lot for reducing the shield, and Fellhounds Reflects make him an absolute Fire Knight hero.

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His A2 here will ensure that every FK attack reduces the shield counter by 5.

After we get the FK's shield down we want to keep it down. A great champ for this would be Armiger, who TM controls with every hit, provided you can get him to 100% crit rate and at least 10 accuracy per level you are trying. (SO FK 15 you will want 150 accuracy)

Then we want to make sure we are clearing the waves. So we want our standard AoE Dec Def in there, and a wave nuker, such as Kael. 

A champ like Deliana could lay down extra damage and help keep us alive with leech.

I might try the following:

Deliana, Kael, Warmaiden, Armiger, Skullcrusher

And if you're not having any trouble blasting through, and want to better control FK shield, you could swap Fellhound in there.

I would also 6 star my Skullcrusher there. He's gonna help you out tremendously in CB and in the dungeons, especially FK. :)