Concept: Solving the Grind: A Solution that Works.

[SuperSpyroDragon64]

Step 1, add a NPC to every Pokemon Center who will ask if you want to battle him. Select yes, and the fight begins. This ensures you don't have to reuse the VS Seeker/wait for a rematch timer to expire each time you want to fight this NPC.

Step 2. Give this NPC six Blissey/Audinos with Parting Shot. Or if this is an earlier-gen hack without later-gen Pokemon, six high levelled Selfdestruct Chansey. Send out a Pokemon you want to level up, switch to a Pokemon you don't mind fainting, he takes the Selfdestruct damage and goes down, and your first Pokemon gains the EXP.

In fangames, you can implement a Level Cap that rises depending on your current number of Gym Badges.

In hacks, you can give each Pokemon Center trainer weak Pokemon with a high EXP Yield and a level appropriate for the player's intended level. So with each new town, the local rematchable Chansey/Blissey/Audino guy will have higher leveled Pokemon.

 

[Jazzy]

Would recommend giving the opposing Pokemon something that doesn't attack the player's Pokemon. Something like Healing Wish would work way better. I say this in mind to nuzlockers who don't want to lose their Pokemon grinding.

 

[SuperSpyroDragon64]

Good idea. Useless status moves that cannot benefit the enemy EXP Grind Pokemon would be great, too. If Parting Shot Blisseys and Heal Pulse Audinos are not available due to the generation, Screech Chanseys could work. Having lowered defensive stats won't matter if the foe cannot attack you.