When you leave Circhester Stadium, having acquired your 6th Gym badge, Sonia will come to see you again. But it's pretty cold outside, so she suggests a better place for a chat. Head west from the stadium, then go down the left-hand side path. Afterwards, turn left to find the restaurant that Sonia mentioned. Inside, Sonia will make a surprising discovery.
Once you're back in control, exit the restaurant. You'll be escorted to the hot baths. The heat from the baths will get to Hop's head and he'll challenge you to a battle here and now. Sonia will heal your party beforehand, so there's no real need to prepare.
Pokémon Trainer Hop
|Inteleon (if you picked Grookey)||41||Water|
|Rillaboom (if you picked Scorbunny)||41||Grass|
|Cinderace (if you picked Sobble)||41||Fire|
Hop has brought back his old team, although some of them have now evolved (and there's one new face). Also, there will be hail during battle, which you could use to your advantage if you have Ice-types.
First up, he'll send out Dubwool, which is an evolved Wooloo. Of the bunch, it's the most straightforward to deal, but it is pretty bulky. Knock it out with Fighting moves if you can.
Next, he may send out Corviknight, which is a fully evolved Rookidee. It's gained the Steel-type and is weak to Fire and Electric only. Offensively, it's not much of a threat, but it can resist a lot of types.
His third 'mon is a new one. Pincurchin is a pure Electric-type, which you can catch very soon. Don't make the mistake of thinking it's a Water-type and don't hit it with Electric moves, because it has Lightning Rod.
Hop's Snorlax has also made a return. As before, it can potentially take many hits, but it lacks a form of recovery (except for a Hyper Potion). So just keep unleashing your moves and it'll eventually go down.
Finally, when you reach Hop's starter Pokémon, it'll be plain sailing. Just sick your own starter on it.
After that cozy battle, you're be all on your own again. When you're ready to leave, head east from the entrance of the hot baths, towards the boutique. Then go down the right-hand side path, where the houses are. When you reach the hair salon on the right, continue going down to exit Circhester for Route 9.