You need to kill two opponents with your 3 ability Searing Flesh. You'll spray a small trail of flames in front of you dealing magic damage.
The ability doesn't do all too much but if you focus on someone low health and you wait to use it till they're very low you should get the kill.
Start with your 1 ability Shackles first which you can hit up to three times in rapid succession if you hit the time before it. This will deal a small amount of damage as long as the chains last and it will prevent them from using mobilization moves like dashes and jumps.
You need to kill two enemies with the flames in a single match. This can be done in any mode, I advise Arena as you have everyone clumped together and it's easier to get to the low health enemies.