First of all: stop making new champs lvl 50. Instead, level up only food champs and make some of your actual lvl 50 champs lvl 60. You will get stucked with lvl 50 champs, like you actually see in Fireknight.
FK lvl 13 is force affinity. If you bring Galek, Kael, Scyl, Alure and Fellhound, 4 of 5 champs have weak affinity for that dungeon stage. They just don't survive the hits from the FK, especially when they are only lvl 50.
For FK 13, I recommend High Khatun (speed up your team, decrease speed debuff on her A1), Vogoth (triple hit A1 to deplete shield and his passive heal to survive, put him on team position no. 5), Armiger (to keep the shield down once it's depleted, this effect needs 100% crit rate and accuracy), Dagger (decrease def, especially to speed up stage 1 and 2 of the fight, and she has some minor tm effect on her double hit A1), Alure (just to good in FK, even with weak affinity, use hp% chest to make her survive).
Stragey is not to let the FK gain turns and use Fellhounds reflect to deplete the shield, but to deplete it with Vogoth's and Dagger's multi hits and keep it down with turnmeter reduction once you got it down. Your champs need to be fast to achieve that.