Li Li is part of the main story, so you will be required to do this sidequest when you come to The Trials of Boddly in Chapter 4. You will find Li Li northwest of the library in Goldpaw, where she will test you by asking you to defeat three Skeleplasms. This is a pretty simple sidequest, since you will be able to find these enemies in the area right outside of Goldpaw. You don’t need to search for actual Skeleplasms on the map, as there is a chance that any enemy encounter will yield these. Once you’ve defeated three of them, then report back to Li Li to finish.