I would use Mean Look. Mean Look can be taught to Gastly, Haunter, and Gengar the earliest, at level 8. If I were you, I would catch a Gastly from the Old Chateau, and then go to the Move Re-learner at southeast Pastoria City. Then, make it hold a Quick Claw, which can be found in a house in Jubilife City (I can't remember which one), so the Gastly will have a 18.75% chance of going first. You could also train it until its fast enough to go first.
~Prof. W.