For this achievement you'll need to feed 50 slimes on the ranch their favorite type of food. These do not have do be 50 different slimes. In order to find the favorite food of each slime, go into the slimepedia. Alternatively, just check out this list!
Pink Slimes - No Favorite.
Rock Slimes - Heart Beet
Tabby Slime - Stony Hen
Phosphor Slime - Cuberry
Honey Slime - Mint Mango
Boom Slime - Briar Hen
Rad Slime - Oca Oca
Crystal Slime - Odd Onion
Hunter Slime - Roostro
Quantum Slime - Phase Lemon
Dervish Slime - Prickle Pear
Mosaic Slime - Silver Parsnip
Tangle Slime - Painted Hen
Gold Slime - Gilded Ginger (Not sure if this counts towards this achievement)
Any largos you may have will accept the favorite food of either of its two parts, counting towards this achievement.
That's it! For a more passive completion of this achievement you can buy auto feeders and load them with the favorite food of the slimes inside.
Here's my video for all achievements if you'd prefer. Skip to 3:50 for Tasty!