WaterGrass
Water and Grass types: weaknesses, resistances and Pokémon
Water and Grass Pokémon take double damage from Poison, Flying and Bug moves and resist Water, Ground and Steel moves.
The balance is even: 3 weaknesses and 3 resistances or immunities.
Water moves are super effective against Fire, Ground and Rock and not very effective against Water, Grass and Dragon. Grass moves are super effective against Water, Ground and Rock and not very effective against Fire, Grass, Poison, Flying, Bug, Dragon and Steel.
To beat these Pokémon, your best pick is Poison, Flying and Bug moves. Avoid Water, Ground and Steel moves, which deal little or no damage.
There are 3 Pokémon with this combination, such as Lotad, Lombre and Ludicolo.