This goal requires you to evolve a TandemausTandemaus into a MausholdMaushold.
TandemausTandemaus can evolve by levelup during a fight starting at level 25.
TandemausTandemaus doesn’t need to be sent out to evolve.
If it is in your party, it might evolve without the evolution animation.
TandemausTandemaus are available for capturing in The Pokémon League and easily capturable with a Pokéball.
Continuing on your run, your TandemausTandemaus will level up and will reach level 25 with enough time and experience points.
TandemausTandemaus are available later, e.g. near Medali.
Capturing one near or over level 25 massively decreases the time you need to spend with it in your party.
There is a static encounter for a Level 35 TandemausTandemaus north of Medali and east of the bridge connecting West Province (Area Three) and Casseroya Lake.