UP IN FLAMES
Incinerate 15 enemies
The Order: 1886 UP IN FLAMES
Load up Chapter 3 / Scene; Rebel Contraband. You will be able to quickly get this trophy in the next section after you picked up the M86 Thermite Rifle in a cut-scene. First shoot the flammable dust (R2) at the enemies, then shoot it with the ignition flare (R1).
Kill all enemies in the building in front of you and back out into the title screen and continue the game. You will start right before the shootout and ramp up more incineration kills towards the trophy.