Easy spot to do this if you have already liberated the outposts is the airport where you rescue Riley in the helicopter. Coordinates are x267 y372 (2nd island)
You can fast travel to 'Krige Valley River Fishing' (east of the airport) and just drive a short way up the road to get there. Come with your fully loaded flame thrower and have at it. There is ammo resupplies for the flamethrower in the North East and North West buildings.
I managed to get my remaining 45 without having to reload or fast travel about, just by tapping the shoot button when a couple feet away to light someone up and running up and down the airport to get the mercs to respawn.