You will get this in your first main battle with Nemesis after you escape the building he us burning with the flame thrower.
I was well equipped, there is a storage box and save just before.
He will have a flame thrower and fuel tank. You need to explode the tank.
Thus won't be the end of the fight, he will keep coming but can be put down with a few shotgun or explosion ammo shots.
Here ia a video if you would like to see it