This one is very simple and can be done nearly immediately. Largos occur when a slime eats a plort that is a different type than their own. The most efficient way to complete this would be to put three pink slimes in a corral, and then shoot one of three different plorts (Rock, Tabby, and Boom are likely to be the first different plorts you get) in with them. Be careful about making Tarrs though!
Here's my video for every achievement if you'd prefer. Skip to 2:50 for Risky Business.
Happy Hunting!