Here is the full list of all 13 Unnamed Fiasco achievements worth 1,000 gamerscore.
All players have died in a short period of time (in 4-player mode).
Win a Dog Tag game with least kills.
Get most of your kills when the Minute Madness is enabled.
Be the player with fewer points at sometime and still win the game (in 3 or 4-player mode).
Do not get killed in a Police Chase game.
Kill at least one player every time you are the police in a Police Chase game.
Perform 3 Triple Kills in a single match.
Retrieve 5 Treasures in a row in Treasure Hunt mode.
Get five kills without using your gun.
Keep the flag for two straight minutes in Capture the Flag mode.
Win by scoring more points than all other players combined (in 3 or 4-player mode).
Be the first one to capture the flag and never let go of it.
Achieve 100% in the single player challenges.