Also need that one to finish the weekly; so far I got Magister, which has a 50% chance of placing a 50% [Heal Reduction] debuff for 2 turns.

All-in-all a crap champ so I'm going to pay attention to this thread as more people reply with more (and hopefully better) champions that can apply that debuff. 

You need to have high enough ACC to land the heal reduction.  The ACC required increases at higher level bosses. I needed around 150 I think for FK18/19.  It is best to have someone with 100% chance to apply reduction.  I also use Alure (TM reduction) to prevent FK from getting shield back up. Then I kill slowly so I can get as many heal reductions as possible in one run.   It counts every time the skill procs even if its already placed. I used grizzled jarl, which isn't ideal because he is not 100% chance. So I never got more than 10 pts a round.

https://ayumilove.net/raid-shadow-legends-heal-reduction-skill-champions/

Jingeon said:

Place heal reduction debuffs on the fire knight 100 times. What characters do i need to use to accomplish this? Or what character skills will work best?

Hi, 

You can use Rocktooth for that. He is a rare that you can get from the Campaign - Felwin's Gate.