Short Answer 
Gengar cannot be obtained in Pokemon Brilliant Diamond without trading, as it evolves from Haunter exclusively through player trades. There are no alternative methods for this evolution in the game, so players must collaborate with friends to complete the process.
Step 1: Understand Gengar’s Evolution
In Pokemon Brilliant Diamond, Gengar is a powerful Ghost/Poison-type Pok‚à ö¬©mon that evolves from Haunter. The evolution process strictly requires a trade, meaning that without engaging in a trade with another player, you won’t be able to obtain Gengar. This evolution method is consistent with the franchise’s tradition.
Step 2: Recognize the Limitations
In this specific game, there are no alternate methods available for evolving Haunter into Gengar. Unlike previous versions of Pok‚à ö¬©mon games, which sometimes offered NPC trades, Pokemon Brilliant Diamond has eliminated these options. Players are left with limited alternatives when attempting to directly acquire a Gengar.
Step 3: Explore Trading Options
If trading is possible, consider involving a friend for an efficient solution. Here’s how you can execute this:
- Trade your Haunter to a friend locally.
- Your friend will see Haunter evolve into Gengar during the trade.
- Ask your friend to trade Gengar back to you after the evolution.
This method ensures you can ultimately add Gengar to your collection while still adhering to the game’s mechanics. Remember, the evolution occurs immediately after the trade is successful.