Shovel Knight: Treasure Trove

This achievement will require you to kill 5 enemies with 1 Mobile Gear. 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 0:40)

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 Clearing a Path, just use the phase ability to lure the enemies together, making sure to lure them over to the left-hand side by the checkpoint, then switch to the Mobile Gear relic. Activate it and jump onto it and back off quickly to send it rolling through the enemies for 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 Clearing a Path at the 17:14 mark.