On Route 6, items are scattered across the land. Some of them may trick you into encountering a Foongus. Each one you mistakenly interact with is on Level 20. Great Balls would be no problem trying to catch it. Later on, if you go to Route 10, you may run into the same problem while trying to pick up items, and the Foongus you encounter will be on Level 30.
Pokémon Black Version (Japanese) – Catching Foongus
