Question

How can a Gastly/Haunter/Gengar learn the move Nightmare before 50Lv? I want it for the Battle Tower please tell me!

Answer 1

Well, you're gonna have to do the following:

• Obtain 2 Gastly in FireRed or LeafGreen. One male, and one female.
• Train them both to Level 53, where they will know Nightmare.
• Head to Four Island, and put the two Haunters into the Day Care.
• The egg they produce will have a Gastly with Nightmare.
• Trade to your Emerald afterward. I suggest you evolve it into a Haunter before trading.

Source

Answer 2

Actually, this is not possible in Emerald. In Fire Red and Leaf Green, it learns Nightmare at Lv. 41. So, overall, it might be difficult to recieve one.