The thing is to just stay in character. If you always use a magic missiles to start a battle but switch to a fire-based spell for a troll, then that's silly. Unless you actually have some reason to know their weakness.

It's tricky because stuff like skeletons being weak to bludgeoning is something that makes sense in a normal sense. So it's not unreasonable to switch to that, but it borders on metagaming. And with other general stuff, it's hard to know how much is known in the world. Does everyone know vampires are resistant to necrotic damage or not? Maybe not townsfolk, but what about adventuring parties? We can't assume that these people have absolutely no knowledge of anything, even the most common monsters of the land.