Things You'll Need
Instructions
Go to Route 8 while it is nighttime in the game. The Pokemon found here differ between morning and night.
Remove the bush blocking the way to the grassy area in the middle of Route 8. The only way to get past this bush is by having a Pokemon with the Cut ability.
Move a Normal-type Pokemon to the top of your list to make it your default when a battle starts. Haunter's only damaging attack is Ghost-type, which do no damage to Normal-type Pokemon.
Walk around the grassy area until you encounter a Haunter. A Haunter appears 30 percent of the time.
Throw Dusk Balls at it every round until you capture it. Even at full health, you only have a one-in-three chance of capturing it. So It's quicker to start trying to capture Haunter from the beginning of the fight, not bothering to weaken it first.