Sorry to disappoint you, but Haunter doesn't evolve by level. It can be evolved at any time after it has become a Haunter, but only if you trade it to someone (another one with a DS not in game). It will then evolve into Gengar and then you trade it back.
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You will need to level up Haunter to level 50 to evolve it in Pokemon Resolute.
You can find Haunter in SoulSilver in Route 8 at night. Additionally you can catch a Gastly at night in the Sprout Tower or the Bell Tower and then you can train it until Level 25 which is when it'll evolve into a Haunter.
Haunter evolves through a trade.
you have to trade
no level you have to trade dude