Those red fuel barrels and propane tanks are actually super useful when youre dealing with big groups, and I've found the best way is to basically create a trap setup. What I usually do is find a good spot with multiple explosives, maybe near some narrow paths, and then get the enemies to follow me through there. Works really well with freaker hordes especially, since they tend to bunch up nicely when chasing you.

Something interesting I learned the hard way - you definitely want to keep your distance when shooting those barrels, and try to avoid blowing up areas where there might be good loot nearby. The explosion radius is pretty big and can destroy nearby containers which is kind of a bummer when youre trying to stock up on supplies.

For those marauder camps, I like to make some noise and get them all grouped up near the explosives before taking the shot. Pretty satisfying when you time it right and catch a whole bunch of them in one blast. Just remember to always have an escape route planned, cause sometimes things dont go exactly how you expect and youll need to reposition quickly.