The False Knight is the first boss you will encounter. He can be found in this room in the Forgotten Crossroads (rudimentary path from Dirtmouth shown):
He will do a few different attacks that are easy to avoid. Counterattack when you have an opening and after dealing sufficient damage, he will fall down and a maggot will pop out of his helmet. Attack the maggot until the knight gets back up (you can use some time to focus heal beforehand if needed). Repeat this process until the knight eventually breaks the ground and falls to the level below, then jump down and attack the maggot to complete the encounter and unlock the achievement.
Note: you do not build soul when attacking the armored knight (but do when attacking the maggot).