Brink

You will need to be Rank 5 level 20 and use one credit to be able to use the Napalm Grenade. From the Main Menu > go to Characters & Weapons > then Abilities. Under Soldier Abilities there will be the Napalm Grenades Ability to the far right in the middle. You will need to use one credit to get this one. You get one credit every time you level up.

Choose any mission to play on and choose the Soldier as your character classes to play as. After the mission starts, press or (Depending if you have abilities already). After you reach a good amount of enemies in your area press the button to throw the Napalm. There will be a fire that will last for a short time. You will need to kill at least one enemy for this trophy to unlock.

The trophy may take a while to pop and if it doesn't try to throw another Napalm grenade again. Shoot the enemy first and then throw the bomb at least that way you know the little health the enemy has left will be depleted once the Napalm goes off.