Done via Skirmish:
Opfor: 1x Urbanmech
Bring 4 firestarters.
When the urbanmech goes into shutdown, there's little point flaming it further as it does next to no damage. Brace until the mech powers up again, then flame it. Therefore you won't run out of flamer ammo. Make sure to only select your flamers when you attack.
It appears that you need to kill the mech with actual flame damage, rather than via overheat damage. You can hit the urbie mech in the rear until there's a few hitpoints in structure left, less than 20.