Suggestion of Shoggoths, the "Thing in the Fog" from Masks of Nyarlathotep London chapter, SO MANY BADGUYS an endless wave.
Werewolves in their human form, still need silver bullets to kill right? How long would that take them to figure out? GHOST Fight!
Have all their weapons get jacked, (stolen or broke), then attack them with bad doods.
If they manage to dispatch your monster(s) and you thought they would surely perish and you really just are going for that TPK, Send in the monsters' friends, masters, servants, summoners, and of course their entourages until all are dead. You are the almighty Keeper, if you want a Cthonian to burrow up out of the ground- just Fire it in there.
Whenever we play Call my players always end up with goddamn 90% shotgun, one of the reasons we switched to Trail. More foes than bullets works pretty great though, and who is to say that once the cultists realize their enemies are olympic fucking marksmen, that they themselves do not recruit bullseye and annie oakley to their cause? "Yeah sorry, he used to be a Green Beret but now an alien is inside his body."