This achievement is very straightforward, and can be quickly completed. You just need to get 6 different types of slimes. You can get the following slimes from the following areas.
Dry Reef (Starting Area):
Pink
Rock
Tabby
Phosphor (Only at night)
Moss Blanket/Indigo Quarry:
Honey/Rad
Boom
Here's my video for every achievement if you'd prefer. Skip to 2:55 for Six Pack.
Happy Hunting!