I know I'm answering my own question here, but it's allowed. This is a way to level grind that I found works, for future reference for anyone who needs it.
You will need:
– a Dungeon with medium-high level Pokémon, and no Ghost-types
– a Mobile Scarf
– three teammates willing to defend the trainee
Okay, so I stumbled across this will recruiting Phione in Miracle sea. I had my lv. 10 Bulbasaur tag along for the ride so that he could maybe gain a level or two. When he came out, he was lv. 18.
I gave him a Mobile Scarf. While hard to come by, they are very useful; they allow you to walk across any surface (water, lava, chasms) as well as through walls. Since Bulbasaur was so underlevelled I decided that I could always tell him to run off through the walls if he got in trouble. While my party was walking through a narrow passage, Bulbasaur went into a wall.
A sudden realization hit me thatmy underlevelled Bulbasaur could hide in the wall, completely invulnerable, while I farmed Exp. for him. I went into his 'Tactics' and changed him from 'Let's go together!' to 'Stay there'. He stayed put in the wall! This is how I level grind now, in conjunction with Silverdragon's answer.
There are two weaknesses to this strategy:
1 – Ghost-type Pokémon can walk through walls and deceasify my Bulbasaur. Stay away from dungeons with Ghosts in them.
2 – Spread moves, like Ominous Wind and Earth Power, can hit through walls. I almost lost Bulba to Lanturn's Discharge.
That is my strategy for level grinding. Please refrain from upvoting the answer, since it is my own. If you feel the need to upvote, go upvote Silver's.