Shovel Knight: Treasure Trove

This achievement will require you to kill an enemy that's just about off screen. King Knight's stage with all the Propeller Rats is a good place to earn this.

Here is a video showing an area of where it can be done. (skip ahead to 1:08)

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.

There is a spot on the Pridemoor Keep stage that is fantastic for knocking out quite a few of these miscellaneous achievements. You'll reach a checkpoint with several of the flying enemies right at the checkpoint location. Using this spot, you can knock out multiple achievements. I suggest coming back after you've gotten all the relics so that you can use the Phase ability to group the enemies together better, then switch to the appropriate relic for each achievement. Leaving the checkpoint/enemies screen and re-entering it will respawn the enemies, and if you run out of magic you can simply commit suicide in the nearby pit to refill.

For Flare Wander, kill off the first group of enemies near the checkpoint. Head to the right and jump across the first pit so the screen starts to scroll right, revealing an enemy. IMMEDIATELY run back to the far-left side of the screen and fire a shot straight to the right. It should hit the enemy just as it comes onto the screen to follow you, unlocking the achievement.

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 Flare Wander at the 17:56 mark.

I did it on the first stage, Plains, with the first enemy (sort of a red beetle).

Go right until you see him and slowly go backwards until you reach the start of the level. You should see him appear on screen, throw a fireball at him. It took me a couple of tries to get the timing right.