Here is the full list of all 12 Gotham City Impostors (GFWL) achievements worth 1,000 gamerscore.
Kill ‘em, fumigate ‘em, and pysch ‘em out… all over Gotham.
Kill 50 enemy players using the panic-slap melee attack.
Accept your first Promotion.
Complete all feats related to a particular gadget or support item.
Complete all feats related to a particular loadout weapon.
Become a Nemesis 50 times.
Turn the tables in Fumigation when the enemy team's victory is imminent.
Earn 50 gold medals in Challenge mode.
Earn a 10-kill streak.
Unlock any locked loadout or costume item.
Finish one match of Gotham City Impostors.
Complete the Initiation mission.