Cuphead

Your "Extra Special" move is the one mapped to your "Ex Shoot" attack (B by default). This changes to your super attack if you have all five cards on your meter (bottom left of screen, gained by shooting enemies and parrying). As a result, you really want to do this before you've earned and equipped your super (gained at the mausoleum).

After you have a super, it doesn't look like you can unequip it. That means that any time you gain all five cards, instead of being able to spam your extra special move, you shoot off your super attack, which won't get you this achievement and drains all your cards.If you've already earned your super, you'll want to unequip it so that you don't fire your super and drain your entire meter when it's full. Unequip it by selecting it and pressing "Y" in the power selection screen. [This hot tip brought to you by TA user ACBL80, who can read better than I can.]

So this is best done right away, at the very beginning of the game. If you do have your super, make sure you routinely fire off your special to use a card and keep the meter below 5 full cards. Otherwise, it's fairly straightforward using the first boss battle ("Botanic Panic," against the Root Pack). It can be done on "Simple" difficulty.



EDIT: TA user JORAX mentions that he had trouble getting the achievement when using the special move for the spread shot, but it popped without issue using the default weapon, so try that if you have issues.

Here is a quick guide on how to make sure you kill the boss with your special move!!

I got this achievement on the Mausoleum I in the bottom corner of Inkwell Isle I - near the fishing "Goldfish girl."

All I did was play normally and parry all the ghosts until the end and POP.