Here is the full list of all 12 Multiwinia achievements.
Kill a total of 1,000,000 enemy Multiwinians
Nuke one of your allies spawn points during a co-operative game
Win a game of Rocket Riot with no rocket damage, before any of your opponents reach 50% fuel
Win a game of Blitzkrieg in under 5 minutes
Capture twice as many statues as your opponents in multiplayer Capture the Statue game (minimum 3 statues)
Defeat someone who is already a Master Player
Win a King of the Hill game with at least 50% more points than the nearest opponent
Capture all spawn points during a multiplayer domination game
Finish defending a game of Assault without giving up a single Control Point
Win a game on every single map available
Play at least 1 game on every map