From Bulbapedia, the community-driven Pokémon encyclopedia.
The Monster Swap Deck is a type deck found in the Pokémon Trading Card Game Fossil Expansion Player's Guide. The deck is classified as one of the tournament winning decks, containing more rare cards and strategy than other easy-to-build decks.
CONCEPTS: Here's a deck designed to make use of two complementary Pokémon abilities. Mr. Mime's Invisible Wall Pokémon Power is almost entirely the opposite of Onix's Harden attack move. Mr. Mime can block the defenders with big damage attacks, and Onix can block all those opponents with the small attacks. Rather than play a guessing game about what the opponent may be playing, use both of these Pokémon to be prepared for any situation.
TIPS FOR PLAYING: Gamblers and Poké Balls are used to set the combo up without sacrificing deck size too much. Neither is entirely reliable so be prepared if the coin doesn't flip exactly like you'd like it. All three main Pokémon components are necessary to properly defend against most opponents, so don't stop manipulating your deck until Mr. Mime, Onix, and Dodrio are all in place.
Once all these monsters are in play, just swap them back and forth as necessary to defend against the opponent and stall your way to the winner's circle. Dodrio is a key to winning with this deck. Multiple Dodrios will provide additive Retreat Aid benefits, and are a good idea with Onix's high retreat cost. Dodrio really allows the "Swap" in "Monster Swap" to shine through.
Mankey and Lass have been added so that you can sneak Peeks at the opponent's hand. With more than one Mankey in play you should get an eyeful every turn! Lass not only will discard the opponent's Trainer cards, but allows you to see what they have left since they must show you their hand when Lass is played. When the time feels right (and using Mankey to constantly Peek at their top Library card will help you know this), use Impostor Professor Oak to toss their hand back into their deck.
PITFALLS: There is a surprising balance to the Pokémon in this deck. Rarely will a weakness issue be your downfall However, you do need to be wary of running yourself out of cards before your opponent. Maintenance has been included in place of the card drawing ability of Bill. You draw fewer cards than Bill, but restocking your draw pile with extra cards from your hand is welcome in this stall deck.