Coldheart is probably the best single champ there is for fireknight, if you aren't running her I'd recommend you do. Lordly Legionnairy was basically designed for fireknight. Other than that it'll depend on who you have. People like Foli, Rhazan, Shazar, and Drexthar are all great candidates for fireknight due to there 3-4 hit default attacks. 

In general look for people who have 3-4 hit defaults and people with heal reduction and turn meter reduction. You can also bring people with counter attack buffs or reflect damage buffs. 

Hope this helps. 

I struggle with this too. 

I found taking champions with multiple hits, on default attack, take the shield number down. retribution effects (skills or gear) really help too, once shield count down then its reducing turn meter.

Armiger, green uncommon is easy to get and level up for this.