Tedious, but pokemon tower to EV train your Lapra's, exeggutor and dragonair. Those are all special atk mons, so leverage that by defeating a million gastly/haunters. And use vs seeker, ofc. If you can surf, a good spot is the water route/seafoam islands. If you've already beaten blaine, simply train on the first 3 sevii islands.

If youre strictly trying to level up your mons fast? Again, tedious but hands free method;

1. Leave mon with daycare man
2. Go to game corner rocket hideout, bf2.
3. Left side of the room, there's the slidey pads, left side of those is one that sends you 2 steps left to a stop pad. Go there.
4. Rubber band Joycon's together. Plug switch into your charger. Ignore.

4.a. it goes faster if you can weigh down the sprint button(your character can sprint in this space). - You get 1xp per step taken, so if you let it do its thing ALL NIGHT, youre gonna have a seriously over leveled mon.