Who are you using now besides coldheart? Duchess stag coldheart visix plus best red/green nuker to clear waves should be safe but faster than whatever you have 

For FK 20, Duchess shouldn't be necessary. The FK will never get a turn, and the waves in 20 don't hit so super hard and Visix (AoE provoke) allready brings a lot of safety.

I suggest Rhazin instead of Duchess. Triple hit A1, decrease def + weaken once the shield is down and 100 tm reduce A3.

Your team has no AoE decrease def and beside Rhazin's A3 no AoE damage. I guess, the 7 minutes the fight takes are only to a small percentage at the boss himself and most of the time at the waves.

I'd replace armiger with Visix. She hits much harder with AoE for the waves, and brings enough safety that you can probably replace apothecary with someone who'll help clear the waves.

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I wouldn't replace Armiger, he is usefull at the boss. But Duchess and Apo shouldn't be needed. Here an example team with AoE decrease def (Deacon) and AoE damage (Magnarr). Visix was the only teammember that got hit at all. Her provoke is safety enough for your team.

Offense is the best defense, dead enemies don't hit you anymore. 😈

@Skadis Revenge 

I used Deacon, Drex, Armiger, CH and Visix.  They ran it in 2:46.  Thanks for your help.