Brawlhalla

As the description states, you must KO 10 people with a slide charged signature attack in matchmade games. Signature moves, which are the moves used with either B or Y by default, can be charged whenever they are used. The way slide charging works is by charging an signature attack while moving towards the edge of a platform and, using your momentum, sliding off of the platform during the charging of the move. This lets you use the signature attack in midair. Note that this is not possible with all signature attacks. The easiest way to do this is by dashing towards the edge of a platform to get the momentum you need and holding down the button before going off of the edge. You must get 10 KOs using this process, which is tricky to do normally, but by utilizing playlists like free for all, it is more than doable. Just pick a character you know how to use that has signature moves that work for slide charging and you'll get it eventually.

If you don't want to do this normally, though, there is an easier way to get this achievement without boosting. Simply go into free for all with a second controller. Not every map works for this method, but just try to hit your stationary second controller and knock them off of the stage with a slide charged move and it will count for one of your 10. Obviously, there will be two other people in the game messing this up, but it's way easier to get these 10 kills off of an afk player than a real opponent.

I've included a clip below of me performing slide charged attacks in training mode. Hopefully this helps those of you having trouble!