This achievement is for beating the first BOSS in chapter 3.
Tips for beating him.
-Collect as many blue orbs up to this point. Check my guide for completing the first blue orb. This will increase your health gauge.
-Jump and slam your sword often and buy the first sword technique where you can charge into enemies.
-He only has a few different attacks so just dodge when you can.
-If you want use devil trigger. Not required though.
The achievement will pop after you get to the mission screen.