If facing a monster or serial killer that knocks off his targets one by one always split up and search the area solo rather than as a group.
The monster/serial killer isn't really dead until the credits roll so don't walk up it's 'corpse' and poke it with a stick. And in many cases it still will return in the sequel.
Most monsters are immune to bullets but can be pierced and killed by an arrow.
Guns can only shoot two or three feet. If you're covering someone with a pistol never stand on the other side of the room, you must always walk over to the other guy and hold the gun about three inches in front of his face (so he can knock it out of your hand if you're distracted).
Anything in the world can be hacked (even if it has no connection to the internet) by punching two or three keys on a computer.
Aliens may be advanced enough to have faster-than-light drive systems and gigantic spacecraft but their weapons are rarely any better than our own at the point in time the movie is made. (Aka a blaster is no better than a M-16, a flying saucer has the same performance as a F/A-18 if today. If made in the '50s the saucers are no faster than a F-86).
Any animal if it becomes huge becomes magically bulletproof.
The US military has special projects to create aliens, werewolvies, vampires, giant animals, dragons, etc as biological weapons.
A 5 foot 1, 90 pound bikini model can take on a half-dozen professional wrestler/bodybuilder type guys three times her size and whip them all.