Slimer is an achievement worth 10 GamerScore in Terraria. It is awarded for killing every type of Slime enemy in the game.
There are 19 varieties of slime in total, many of which you will encounter through regular play. Here are a list of all 19:
Slime Name|Location|Hard Mode Only?
Green Slime|Spawns in Forests.|No
Blue Slime|Forest Underground.|No
Red Slime|Underground layer.|No
Purple Slime|Spawns in Forests.|No
Yellow Slime|Underground layer.|No
Black Slime|Cavern layer.|No
Mother Slime|Cavern layer.|No
Baby Slime|Spawns when a Mother Slime is killed.|No
Pinky|Forests and the Underground layer. Rare, so may take a while to find!|No
Jungle Slime|Spawns in the Jungle.|No
Lava Slime|The Underworld.|No
Dungeon Slime|Indigo coloured slime; spawns in the Dungeon.|No
King Slime|A large blue boss slime; spawns in Forests away from your spawn point. You may summon one by using a Slime Crown (Gold Crown and 99 Gel at a Demon Altar).|No
Corrupt Slime|spawns in The Corruption/ Underground Corruption. Dark blue in colour.|Yes!
Slimeling|Spawns when a Corrupt Slime is killed.|Yes!
Shadow Slime|Spawns in The Corruption/ Underground Corruption. Dark blue in colour.|Yes!
Slimer|Spawns in The Corruption/ Underground Corruption. Dark blue in colour with wings. Kill the wings, then kill the Slime itself!|Yes!
Toxic Sludge|Underground layer. Green in colour.|Yes!
Illuminant Slime|The Underground Hallow. Purple in colour.|Yes!
As noted previously, most of these should be fairly easy to find and defeat while playing through the game. The only one that may prove difficult for some is Pinky, which is a rather rare Slime!
Keep at it, and you’ll find yourself with another achievement… Good luck!