RPGMemes

Metagaming is fine, actually.

To some degree, this is why Knowledge Checks exist. If you're going to Troll Canyon and you make your Know(Local) check to have an idea about what a troll is and does and you get a high enough roll, you know. If you don't, maybe you forgot. Maybe trolls aren't common to your neck of the woods. Roleplay your reasons.

That said, I believe DMs reserve the right to mix it up a bit. As an anecdote, I had a friend play in a game in which they were hunting a White Wyrm in the glaciers of the north. The experienced players, knowing that White Dragons breath frost, fully stocked up and pre-buffed with anti-cold gear. When they arrived, they positioned themselves on a large ice-flow and pushed off towards the mouth of the cave. But the cracking of the ice awoke the dragon. Dragon came flying out, spotted the players, and immediately engulfed them in a plume of fire. The ice flow melted, the party floundered in the freezing water, and two of them died to a happy dragon who'd just been offered an easy meal.

The players were initially upset, but the DM tisk-tisked. "Everyone knows that dragons breath fire".

If you want to play let’s pretend with dice, that’s fine. just be honest about the kind of game that you’re running from the get go so I know not to join your table.

If you're not playing "Let's Pretend" with dice, I'm not sure what kind of D&D game you're actually playing. A dumb-as-rocks barbarian should presumably see the troll as some big meat sack to be repeatedly bludgeoned into a fine paste. And that may possibly work, at least to the degree that the threat is neutralized for the purposes of the combat. A savvy Bard probably has a song or two about the proper remedy for persistent trolls - and a clever player might even dash off a cute little poem or song to help the rest of the party recall.

The dice keep the game spicy, but you shouldn't be shy about leaning into the cinematics of the situation.