Get Armiger and build him up, or try to get Alure ... both are essentially to keep the shield down.

Other good options, of the heroes you already have, are Razin,  Deacon and Lyssandra 

In the stage you are right now, Tayrel will have problems to land his debuffs due to his weak affinity vs. force. You could put in Deacon Armstrong instead (should be lvl 60 anyway): double hit on A1, turnmeter manipulation + extra trun on A3 and AoE (for the waves) def debuff on A2.

Lyssandra would be an interesting option vs. FK, too. But you wouldn't have def debuff anymore if you use her instead of Tayrel/Deacon. Maybe Reliquary Tender is not necessary? Or do some of your champs die often and you need the revive?

Turn  Meter  Fill  +  Increase  Atk --->  Old  Hermit  Jorg

AOE  Defense  Down  --->  Tayrel,  Deacon,  Stag  Knight,  or  WarMadien 

(The  hero  you  pick  Depends  on  the  Affinity  of  the  Fire  Knight)

AOE  Nukers  ---->  2x  Coldhearts

5th  hero  is  Flex  Spot:

If  you  find  the  team  dying  to  much,  you  can  run  a  hero  who  does  shields  or  heals  to  keep  the  team  alive  longer  

(Vrask,  Vogoth,  Lordic,  or  Zelotah  are  possible  options)

If  you  find  the  team  is  doing  fine  with  the  core  of  4  I  mentioned,  you  could  run  a  hero  in  the  5th  spot  to  win  the  fight  faster

(Lyssandra  or  Apothecary are  possible  options)

Thank  you  guys  that  is  really  helpfull.  I  will  focus  on  that .  doni  need  to  book  the  champs ?

You shouldn't need to, but coldhearts are easy to book.  Make sure the coldhearts are at 70% crit rate and high of accuracy as possible for later on.  They should cruise through the boss.

If you're still struggling with his shield, take in Apoth.  If you're failing to keep his turn meter down once his shields are off, take in Lyssandra or armiger.  If you're struggling with the waves, take in a healer like Player J mentioned.

Really with 2 coldhearts you have this made.

I disagree. I think aoe def down isn't needed. You have Razhin. He has aoe tm(cc) and boss dec def and weaken. second slot Razhin. This should let ur coldhearts hit wa  harder. A lot of yt use Razhin.

No offense Riptide, but your comment is really not applicable at higher tier levels. Without AOE def down, the waves take waaaay longer to clear.

Kramaswamy is  correct.

AOE  Def  Down  is  needed  to  kill  the  waves  faster  in  higher  stages.

The  problem  you (Riptime) have  is  you  don't  understand how  damage  is  calculate.

You  have  had  this  same  problem  in  other  area's  of  your  game  play  such  as  Arena.

The  game  has  a  Formula which  calculates  how  damage  is  done  against  rival  enemies.

AOE  Defense  Down  reduces  the  enemies  defense.

It  is  a  massive reduction  which  allows  your  heroes to  do  more  damage  against  enemies.

Rhazin  +  2x  Coldhearts  will  do  less  damage  vs.  AOE  Def  Down  hero  +  2x  ColdHearts

Rhazin  is  a  great  hero,  but  he  can't replace  the  AOE  Def  Down  slot.

Rhazin  could  be  in  the  5th  hero  Flex  spot.

(If  the  OP  was  able  to  win  with  the  core  set  up.)

Or  the  OP  could  decide  not  to  use  Rhazin  for  Fire  Knight.

The  OP  could  decide  to  use  Rhazin some  where  else.

searched how to beat fire knight. 1st vid HH, used RHAZIN. Ok. aoe def down can be need. Razhin is also needed for faster fights.

ok. Let me get this straight: my enemies about 40% dmg reduction in arena, aka my elhain does about 60% damage. Even with 60% dec defence, and even if that somehow removes defence, my elhain does more damage from 2 smaller nukes.

Just because he's used, doesn't mean he's optimal. His kit is really not geared towards FK, with the exception of the 3 attacks on his A1. Running it on auto, you'll find his A2 is wasted most of the time against FK's shield, and on waves, if you don't have an AOE def down, you're just going to take forever clearing them.

I have no idea what you're trying to say here. There is almost nothing else in the game that could possibly compete with knocking off 60% of their defence. The only exception I can think of is if you're using moves that ignore defence completely. Otherwise, literally no other buff or debuff will outperform it.

You seem to have no idea how damage calculation in Raid works.

1000 def block 41,4% of damage allready. 2000 def block 72,6% damage. Why do you think your enemies have about 40% damage reduction (that would be less than 1000 def) in arena? Do they fight with ungeared champs?

How far in dungeons are you, Riptide? Did you ever have to fight vs. waves in stages 20+?

HH is fighting level 20 in that vid, you can get through the waves fairly quickly and focus on the boss.

my FK25 team normally takes around 8 minutes and at least 6 of those minutes are spent on the waves.  in the higher up levels, the waves are the challenge and cutting minutes off your runs is important, which aoe def down will do.  rhazin will place 1 dec def buff every 4 turns, and you have 10 enemies in the waves to get through, which means 40 turns to land it on them all if you get lucky and land 100%?  the enemy will be beating on your team for all 40 turns, and an aoe dec champ like stag knight or tayrel will land 50 debuffs in that same time.  there's just no comparison here, and on top of that, rhazins slow attack animation will add minutes to your run by itself.

The OP is trying to get past FK13, sure rhazin can do it.  He's probably good up to 17 or 18, maybe even 20 if you don't care about your times.  After that, you'll need to go a different route, and Player J gave solid recommendations which could work through FK25 once the champs are built up, and most of them will be used in other areas of the game as well.

Now, rhazin is a good champ for many reasons, go ahead and use him. he can be a damage dealer, he provides some crowd control, and weaken/dec dmg for bosses, and 3 hits on his a1 is perfect for FK.  Look for a good AOE def down champ though, they'll be a big help getting you through the waves so you can actually get to the boss fight.