The shadow reaper has really high dodge rate and defense making it a pain to even land a hit and when you do, it does pitiful damage. He has 2 attacks, the first does damage proportional to your current health. The other does about around 7 damage and heals him for 30, provided you are equipped with all the ultimate equipment at this point.
The proportional damage attack is nothing to worry about, even though it seems to deal high damage when your health is full. The truth is, the attack deals damage equal to about 1/3 of your health. So if you have ~20 health left, the attack can only do around 7 damage. Due to the design of this enemy, you will find yourself in low health constantly. That makes the Avenger skill the perfect counter for this enemy, as in the few turns the shadow reaper has you at low health, you would have enough time to take him out with the skill.
He is the only enemy in the game who can steal life from the player to heal himself.