To earn this trophy, you need to kill 15 enemies by incinerating them with the M86 Thermite Rifle. To save time, and because it is the earliest/fastest way to do so, you can also aim for 40 science kills (which the incinerate kills count as) total, 25 more than you need for this trophy, to unlock:
There is no tracker for fire/ science weapon kills.
Instead, just go to the Main Menu, select Chapters, scroll to the right for Chapter 3 - "Inequalities", then scroll down that list to the "Rebel Contraband" checkpoint and load from there.
Watch a few cutscenes before the action, but you come equipped to the fight with an M86 Thermite Rifle. Read the brief tutorial of how to use the gun. You shoot a cloud of ignitable gas/powder with
to surround the enemies. To ignite the cloud, press
to shoot a ball of fire that will erupt the cloud and those inside it in flames
incinerating them for the trophy, and earning you a science kill for each death towards the bonus trophy.
If you don't reach your 15 Incinerate kills (or if you're smart and want to get 25 more to earn your 40 Science Weapon kills) during your run, just kill every enemy in the area until the middle barn blows up, then pause and reset the checkpoint. Repeat until the trophy pops. It make take 5-6 runs. (Restarting checkpoint does not lose track of kills and you only have to watch the last 5 second scene before the action, not all the previous ones).
Please vote if you found this helpful, and please comment if you have any additional insight or questions. I always reply within 24 hours. Feel free to add/ friend me on TT, TA, PSN, or XBL, username: Danokitty. Happy to help!
Happy hunting!
-Dan