Hi achievements hunters !
This achievement may seem hard, but i have found a way to do it really easily.
First of all, of course, you have to unlock the lab station (10k coins) and buy the slime basket (7.5k coins) and finally creating it with the necessary components.
Once you have it, go to the first area of the game (dry reef) and at the very beginning of this area, on the left, there is a spot where you can put your slime basket.
Once it it placed, there is a small area in front of it, on the left, where you can climb with your jetpack, and now since you are higher than the basket it's very easy to put 50 slimes in it like i did in this video. Just be sure that you have a lot of slime in your inventory in the first place (there is a spawn point just behind me in the video).
I was able to make 65 on my first try, you can probably do better