I've just reached Mega Gengar now but haven't tried him yet.
What should I do/use?
Disruption Delay, Moves + 5 and Mega Start is usually a good combination of items to choose, and it comes to 4300 coins which isn't that expensive considering you can grind the normal and special Meowth stages for more coins.
It's been a while since I played that stage but I think bringing Haunter is recommended as well as Sableye + 2 other supereffective Pokemon.