I am arguing that Bessemer has a bit of experience having already been owned by a gym leader rather than gaining these levels for one-shotting a Zubat.

Level-Up Form
Story: Resurgent Red Version
Character: Jonah Yermolov
Pokémon Leveling Up: Bessemer the Aron
Starting Level: 10
Final Level: 15
Link to the GCeA Post This Happened: Beep.
Link to stats: Boop.