Haunter evolves by trading it to another player in Pokémon Brick Bronze, and then getting it traded back as Gengar.

Quick Scoop

  • Put Haunter into a trade with another player.
  • When the trade completes, it evolves into Gengar.
  • If you want to keep the Gengar, have the other player trade it back right after.

Useful note

Some Brick Bronze-style fan games also use a Link Stone instead of trading, but the source I found for Brick Bronze itself points to normal trading as the method.

Simple example

  1. Find a trusted friend or second account.
  2. Trade your Haunter.
  3. Let it evolve.
  4. Trade the Gengar back if needed.

TL;DR

Trade Haunter to evolve it into Gengar.