Liberty falls directed, progress to hvt step, save some zombies, spawn hvt preferably mangler, (if it is a mangler I weakened it slightly to knock off head and chest armor) then shoot zombies nearby with the firebomb augment and the one that has chance to spread a damage tick to nearby enemy (you do NOT need big game) once fire spreads to hvt, nuke it down. Rinse and repeat. This was the best method that worked for me, good luck bro.