CapnZapp
Legend
In short, if one of my players ever tried to use an at-will fire to cancel out regen without getting word from me that it was okay for his character to do so, it would be stopped immediately.
I agree with UG - asking the party to not use player knowledge is awkward if that gets them killed, and forcing them outright is not fun. Then it seems to be a better solution to change the damage types around.players who've played a lot of D&D know what trolls are. ... True, you can require the characters to make checks before they can act on this knowledge, but that's not fun. It's not fun for players to ... purposely not use the ability they would have used because their character doesn't have the player's knowledge.
The reason I came to think about Trolls was the P1 module. While generally a useful tool in the DM arsenal, reskinning doesn't really work when the entire module is built on the assumption the Trolls are... Trolls.
I still can't say anyone have really brought up the crux of the issue:
1) even if the players don't know about Trolls, it's just a DC 15 Lore check away (which is trivial at early Paragon levels)
2) wielding fire is trivial too at that level. Even if somehow you can't afford the 1000 gp for a Flaming weapon, there's Alchemist's Fire or even mundane torches to be used. Acid attacks (that isn't encounter or daily powers) is comparatively much more difficult to get hold of. (There isn't any acid weapon enhancements like Flaming in the PHB, and there's no commonplace acid attack like wielding a torch either)
Still, neither the MM nor P1 seems to be written with adventurers automatically wielding acid/fire in mind. As I understand it, trolls would flee when faced with acid/fire attacks? That simply isn't practical, especially in a module like P1.
There's something here I can't fully wrap my mind around...