Haunter evolves into Gengar via Trading.
Trade Haunter with any Pokemon to make it evolve!
You have to trade to another game to make it evolve.
You can get gastly and haunter in lavender town. Trade a haunter to a friend to make it evolve into gengar.
there is no possible way to make him evolve with out trading. you have to trade to make kadabra, haunter, and many others to evolve
Level 25 into Haunter, then trade to make it Gengar
trade a haunter to one of your friends, then make them give it back, or get a haunter from someone else. NOTE: it wont evolve if its holding an everstone.
The way to evolve Haunter has been, and will always be, by trading. If you have a Haunter in Pokemon Emerald trade it to Sapphire, Ruby, Fire Red or Leaf Green and make sure he isn't holding an EVERSTONE as that will prevent its evolution.
In Lavender Town there is a tall building known as the Pokemon Tower. Catch either a Gastly or Haunter, I would catch a Gastly (if you don't have one already) and make sure it evolves into Haunter. Once you have a Haunter trade it with someone, and it will evolve. Then just trade back and you got yourself a Gengar! Good Luck!
Trade it to to somebody else
You don't need to get a gastly to evolve because you can catch haunters in Pokemon tower
Either level 24 or 25. It evolves into Haunter but then if you want a Gengar, you have to trade with someone and then it will evolve and then trade it back. (Make sure they give you a good Pokemon as collateral so you get it back!)
You trade him but make sure he isn't holding anything