Mercy is encountered in the second act of Wolcen Lords of Mayhem.
You will come across Mercy during The Avenger mission. This page has information about his attacks and the most effective strategies.
The boss is accompanied by allies right from the beginning of the fight. Get rid of them first, so that they don't interfere with the fight.
The boss moves quickly around the map by using jumps. He also uses this skill to stun you.
Mercy is also able to spin rapidly with his weapons. Thanks to this attack, he not only deals huge damage, but also moves quickly around the map.
Wait for the boss to get weakened to deal as much damage as possible.
Mercy is a strong and fast opponent. It is better to avoid his strikes and attack him in a such way that would allow you to stun the boss. When he gets stunned, deal as much damage as possible to quickly defeat the boss.