The design behind 12M is simple; there are 12 Messengers. By playing Draw-Go for the first part of the game, you can eventually drop a Messenger and copy it with as many clones as you can.

For sideboard options, there's a fair bit of hate for reanimator and pod (Grafdigger's Cage, Nihil Spellbomb and Surgical Extraction). Flashfreezes are the generic control sideboard card, Ratchet Bomb is for killing spirits and Despise is just kind of useful.