One type is ground, another is water and psychic will work too! If you tell me where you are in the game and what game it is I'll tell you how to get a Pokemon to defeat it with. If it's Colosseum use espeon leave it in day-care and do a load of random battles or do Mt. battle. I know that that armaldo is hard so just a guess.
Ground type is super effective against electric, fire and rock type Pokémon.
Ground-types are super effective against electric-type Pokémon.
Actually the Ground is the only super effective type against thunder type pokemon.
ground type Pokemon are super effective against poison type Pokemon.
Rock, electric and ice are super effective against flying type pokemon.
Bug Type and Fighting Type moves are super effective against it.
Ground type is super effective against electric, fire and rock type Pokémon.
normal type Pokemon are not super effective against any type.
Ground-types are super effective against electric-type Pokémon.
Actually the Ground is the only super effective type against thunder type pokemon.
ground type Pokemon are super effective against poison type Pokemon.
Rock, electric and ice are super effective against flying type pokemon.
super effective is bug
Ground and psychic type attacks are both super-effective against a poison type pokemon.
Steel is super effective against rock type pokemon.
Rock, electric and ice type moves are super-effective against flying type pokemon.
Ghost and Dark attacks are super effective against Ghost type pokemon.