You'll need to find or discover the following minifigures to do this:
Chicken Suit Guy
Lord Vampyre's Bride
Mr. Good and Evil
EDIT: there used to be a Bat Monster minifigure that was part of the achievement, he's no longer unlockable. Thankfully, he's not necessary any more, so the achievement lives on!!
Bat Monster doesn't appear in the game world, you have to unlock him a different way. Load up your save game. From the galaxy map screen hit cn_B to return to the Main Menu. You'll notice that a Lego model is being shown in the center of the screen. You can save these models for use in-game! Scroll to the right until you reach the " + " symbol. Select this to save the current model to your in-game inventory. You can scroll through them with cn_LB and cn_RB if you aren't interested in saving them all. The one you specifically want to save is the Monster Graveyard. Once you've saved it, load up your Monsters DLC world, find an empty space, and "build" the model using the Discovery Tool. Once it's finished building itself, you'll notice a new minifigure hanging out inside. Step into the graveyard and give the Bat Monster a Red Apple. You can now "discover" him and use him for the achievements, too!
If you discover all of the minifigs, you can place them in the world. Either way, shoot them all with the Paintball gun and the achievement is yours!
*if you're missing the Paintball gun, it can be found in the base game. I'd suggest trying the Junkyard Jungle Biome or Falling Forest, the paintball players live there.