Phantump's best moveset in Pokémon GO is Tackle and Shadow Ball, with 9.34 damage per second, and it can deal 107.1 damage before Phantump faints. This is true when battling in neutral weather.
All Phantump movesets ranked
Here is a complete list of Phantump movesets in Pokémon GO, ranked by their DPS and TDO. You can change the weather and the opposing Pokémon.