Phantump is a a Ghost & Grass type, with an okay design. It's not one of the best ghost nor grass type designs out there for sure, but also not one of the worst. Now in terms of how good Phantump is to use in a playthrough, I'd say it could do okay as it does have okay abilities, and an okay movepool, stats could be a bit better, but if used properly, can do justice until it evolves. This isn't something I would recommend to use though as your grass or ghost type of Kalos, as I think there are others that can do the job better, but reason I ranked Phantump here was just due to appeal.
