You need two things to beat the Fireknight. 1. Multiattacks to bring down his shield. 2. Turnmeter control, to keep the shield down once you did no. 1.

For task no 1. you have: 

• Vogoth. Triple hit A1, passive heal, good affinity, helps vs. waves with his A2.
• Apo. Triple hit A1, speed buff, heal, downside: bad affinity for stage 20, should probably be 6* to survive the fight.
• Gala. Multi hits, neutral affinity.

For task no. 2 you have only Armiger. Soulbond Boywer is not good in this fight on auto (what is required for the Arbiter quest), she fires here A3 into the shield.

Maybe the last team spot should go to a reviver (Rector Drath) or a decrease def champ to speed up the fight vs. waves (Valla).

You need multi-hit champs for FW20. Zavia has three on her A1. Peydma has two, which isn't ideal, but isn't terrible. Vogoth has three. Drath can be your healer/reviver if you need. The last piece should be someone with 100% heal reduction debuff. Usually people use Coldheart, not sure if you have her. If you don't, try to find someone else who does.

zavia, vogoth, fodbar, apothecary, and either scyll or rector drath.