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