Luma

If he’s not showing up at the cemetery, you probably missed talking to him at the Tanzan Base. In that case you will need to wait until after badge 16 when you reach the Glass Workstation Base, where he should show up.

Hello :) Lumi and Eve are Ice and Psychic trainers, since 5/6 of your team resist one of those types (which becomes a weakness on inverse field) that is probably why you are having trouble. Try to use Pokémon that are weak to those two types if you can, or try changing the field to flip the matchup (if your sylveon has misty terrain for example). The game makes a backup save before you enter, so if you are really having trouble you can find that save in the saves folder and train some new Pokémon.