So demogorgons are immune to bullets but not swords or wine bottles?

Its kinda weird how they dont seem to care on making their durability somewhat consistent, on 5x4 we saw one tanking literally hundreds of bullets without almost any harm while hopper could literally decapitate one with a sword that had probably lost all sharpness after being used by dozens of prisoners before, and yes, I know it was weakened by the flamethrower but it still doesn't make any sense since the flamethrower only damaged its exterior.

I don't get why the writers didn't simply add one or two demogorgons actually dying from the shots in this last episode, it would make them feel more real. Plus, why do the soldiers only have one flamethrower? Shouldn't they be more prepared to fight the only enemy they're supposed to be protecting the base from?