Here is the full list of all 12 Gotham City Impostors trophies - there are 4 bronze and 8 silver.
Accept your first Promotion.
Kill 50 enemy players using the panic-slap melee attack.
Complete all feats related to a particular loadout weapon.
Become a Nemesis 50 times.
Complete all feats related to a particular gadget or support item.
Earn a 10-kill streak.
Win each game mode in Ace Chemical, Amusement Mile, Crime Alley, Gotham Power and The Docks.
Earn 50 gold medals in Challenge mode.
Turn the tables in Fumigation when the enemy team's victory is imminent.
Unlock any locked loadout or costume item.
Finish one match of Gotham City Impostors.
Complete the Initiation mission.