Shovel Knight: Treasure Trove

Once again, head to Pridemoor Keep. After you fight your first Gryffin, you'll be in an area with around ten- twelve rats. Just lure them in and use the War Horn. If you don't plan on beating the level after you get the achievement, don't worry about dying because you'll go back to the map with what you started with anyway.

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 Boom, just use the phase ability to lure the enemies together, then switch to the War Horn relic and activate it. Easy peasy.

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 Boom at the 16:46 mark.