WaterGround
Water and Ground types: weaknesses, resistances and Pokémon
Water and Ground Pokémon take quadruple damage from Grass moves, resist Fire, Poison, Rock and Steel moves and are immune to Electric moves.
With 5 resistances and immunities against 1 weaknesses, it is a very solid defensive combination.
Water moves are super effective against Fire, Ground and Rock and not very effective against Water, Grass and Dragon. Ground moves are super effective against Fire, Electric, Poison, Rock and Steel and not very effective against Grass and Bug.
To beat these Pokémon, your best pick is Grass moves. Avoid Fire, Poison, Rock and Steel moves, which deal little or no damage.
There are 9 Pokémon with this combination, such as Wooper, Quagsire, Marshtomp and Swampert.