From Bulbapedia, the community-driven Pokémon encyclopedia.
AMU was a semi-popular Pokémon Trading Card Game deck archetype in late 2008. AMU stands for Azelf, Mesprit, and Uxie: the three key Pokémon in this deck. The deck's strategy centered around getting Mesprit LV.X, Uxie LV.X, and Azelf LV.X in play as fast as possible and consistently doing 200 damage with Mesprit LV.X's Supreme Blast attack. The deck faced severe consistency problems and, although it saw a small competitive play revival with the release of Level Max in the Platinum expansion, AMU was a short-lived and fairly unpopular archetype.
Strategy
The strategy of this deck is to repeatedly use Mesprit LV.X's Supreme blast attack, which does 200 damage provided there are both an Uxie LV.X and an Azelf LV.X in play. When using this deck, a player uses Trainer Cards such as Roseanne's Research to search for all of the Pokémon necessary and their non-LV.X forms. The most common way to make this deck is to have 3-1 lines (3 Mesprit, 1 Mesprit LV.X) for each of the three critical cards. However, 3-1 lines mean that the deck completely relies on none of the LV.X cards being in the prize cards, so some people take the more expensive route of 2-2 lines. The deck also often includes many Trainer cards to search for Basic Pokemon and something to knock out a Mewtwo (Mewtwo LV.X can block all damage from Psychic-type Pokémon). Once the player has all of the LV.X cards in play and any Mewtwo the opponent has are knocked out, they will repeatedly use Mesprit's Supreme Blast attack until they have taken their last Prize Card.