Dead Cells

To unlock this achievement, you'll need a weapon or shield with a knockback effect. I personally went with the Gilded Yumi bow, which is a new weapon in the DLC pack. To get the blueprint, you need to kill Euterpe (the bow user among the Queen's three servants) when you face all three servants in the final phase of the Lighthouse chase, but she has to die last. If you kill either of the other two last, you'll get their blueprint instead. Get the Queen to low health where damage from a fall will kill her, then lure her over to the edge. A good way to get her to stay still at the edge for a few seconds is to deliberately fall into the void yourself. When you respawn back on the platform, the Queen won't move for a moment, giving you time to use your knockback weapon/shield/item without having to deal with her going ballistic. If the fall damage kills her, the Beheaded will instantly kill her without the "End her" prompt appearing. That's when you know you got it.

A successful kill: