FirePoison
Fire and Poison types: weaknesses, resistances and Pokémon
Fire and Poison Pokémon take quadruple damage from Ground moves, take double damage from Water, Psychic and Rock moves and resist Grass, Bug, Fairy, Fire, Ice, Fighting, Poison and Steel moves.
With 8 resistances and immunities against 4 weaknesses, it is a very solid defensive combination.
Fire moves are super effective against Grass, Ice, Bug and Steel and not very effective against Fire, Water, Rock and Dragon. Poison moves are super effective against Grass and Fairy and not very effective against Poison, Ground, Rock and Ghost.
To beat these Pokémon, your best pick is Ground moves. Avoid Grass, Bug, Fairy and Fire moves, which deal little or no damage.
There are 3 Pokémon with this combination, such as Salandit, Salazzle and Iron Moth.