Create a corral and collect twenty of any slime - this is easiest to do with pink slimes, as they are all over and right outside the ranch - and deposit them into the corral, get another twenty and just walk in and the achievement will pop.
Note: I found this easier to do after upgrading my Vac to hold 30, instead of 20, and there is also an upgrade to hold 40 of one object - this may be easier for other people.