First off, I suggest going into options and switching your relic activation to Button mode so that you only have to tap
to use relics rather than having to use the left thumbstick/d-pad and button combo. It makes the entire game much easier IMO.
This achievement is super simple. Just play through the Plains stage (the very first mission) and fight the Black Knight at the end any way you wish. When he gets down to half of a circle of health left, wait for him to fire one of his purplish-pink balls of flame at you and reflect it back at him with a swing of your shovel. The silly tin can doesn't even try to dodge it, so it will finish him off for the achievement. This could also be combined with any of the other achievements in the video that are done in the Plains mission, including No Damage, but IMO it's not worth risking ruining your No Damage run waiting around for him to fire at you and potentially missing your swing.
In the interest of creating as much of a one-stop-shop as possible, I've included 22 of the miscellaneous achievements in Shovel Knight in the video below. I wanted to include Knuckle Down as well, but the stage for that was already complete in my game and I haven't played back through again to unlock it. You can see a fight against the Black Knight polishing him off with a taste of his own medicine at the
03:37 mark.