Fenshi have skills built for Fire Knight. His a1 sith decrease speed will help your team a lot. I don't know how well your champions are built, so I will just suggest to replace one of your damage dealers and see which will give you best results. 

I don't have Geomancer, I don't know how well his passive does in fire knight, so I will try to replace  him first with Fenshi. 

Vrask is generally not good in Fire Knight. If you have Apothecary, and have enough tankiness and speed (around 200 should be your target goal) , he can be in your team up to fk20 and above. 

A good strategy in deciding whether to level 60 a champion, is to try him first at 50. If he is already doing well, and helps you push further in a dungeon. Then when you need him more additional stats, then make him your next level 60.

I think he can help you with the upcoming shadowkin faction crypt, but his kit is more towards fire knight boss. 

Armiger is great vs. Fireknight.

Geomancer can steal the turnmeter, but his passive doesn't hit the shields of the Fireknight. He's still good with a 100% turnmeter steal on a (booked) 3 turn cooldown.

Roric is ok with a double hit A1 and a 75% turnmeter reduce on a 3 turn cooldown, but he's not the best FK champ in the game.

Elhain has a double hit on her A1. That's not really good enough. 

Vrask has literally nothing that helps with Fireknight. You shoudln't need his heal, the thing with Fireknight is, that he should never get a turn, or only 1 turn, if you do the fight on auto and your champs join with cooldown on their skills. You don't need heals when you don't get any damage.

You should put in Fenshi and High Khatun instead of Elhain and Vrask. High Khattun is good vs. FK, increasing speed and your turnmeter has a similar effect than decreasing his turnmeter over the fight. Armiger, Geomancer, Fenshi and Roric can use their tm-skills more often.

Mostly agree, but regarding Elhain, he probably needs her for the waves as well. Considering that all of Elhain's skills are double hitters, if her crit rate is good, she's better than most other early-mid game champions to fill the wave nuker role while contributing on the FK himself as well. 

 Yeah, spot on about Elhain. 

 From observing the game it does look like Geomancer's passive can register hits on FK's shield but only when the giantslayer-equivalent effect is triggered. Other than that yeah, he's mostly there for Quicksand Grasp.

 Vrask's job at present is just to keep my team alive through the waves but he's in relentless gear and has revenge accessories so he gets multiple hits in fairly regularly. He might be replaced with another healer that has a consistent multi-hit A1.

 High Katun is getting six starred for clan boss and arena next, I want to try her in Spider too.

 Honestly Roric seems more like an epic than a legendary but he does good work, the main problem is he dies easily. It'd be great if he got a survivability buff.

I am not sure if Fenshi is that helpful for the OP for Fire Knight dungeon.

During the Brogni fusion I had not enough ressources to do the full fusion, therefore I was only able to get two of the four epics and Fenshi was one of them. My idea was that Fenshi will help me to progress in Fire Knight, but in reality this was unfortunately not the case.

His kit is solid for the Fire Knight itself, but not such much for the wave content. For higher stages of Fire Knight those waves can be tough and you need a team, which can handle both waves and the boss. This was my problem.  I had a good team for the boss, but struggled with the wave content. I had to give up my plan to use Fenshi in my team composition, as he did not offer any crowd control abilities, which would be helpful for the wave content.

I am not sure if Fenshi will help the OP that much in Fire Knight. If Fenshi is used it has to be ensured that the rest of team can handle both enemy waves and additional has some healing capacity so that Fenshi is still alive at the boss. He is a little bit squishy in my opinion so take that into consideration. Armiger is defensed based and is therefore generally can sustain more damage than Fenshi.

To sum it up I like Fenshi and his kit is good for Fire Knight, but only in the right team. Honestly, I do not see that the addition of Fenshi will improve OPs Fire Knight team significantly.

When  I  think  of  Fire  Knight,  I  think  of  4  different strategies:

1  -  Counter  Attack  Buff  Set  up

2  -  Reflect  Buff  Set  up

3  -  Rally  Attack  Set  up

4  -  Fast  Multi-A1  hitting  Set  up

The  first  question I  have  to  ask  is  do  you  have  any  of  the  above things?

Do  you  have a team  Counter  Attack  Buffing  hero?

Do  you  have  a  team  Reflect  Buffing  hero?

Do  you  have  a  team  Rally  Attack  hero?