Theo R Cwithin
I cast "Baconstorm!"
Generally I try to make it clear through in-game hints and DM body language that they're in over their heads.
But if the PCs insist on charging ahead, well... let the dice fall where they may.
The caveat: I'll try to avoid a TPK (or allow an immediate rematch) if it's the DM's fault, like forgetting to mention a critical piece of information. This is important to me since I don't put a lot of emphasis on mechanical balance; if the PCs are heading into a situation where they're outmatched, I try to make sure they understand that before it's too late (or at least make sure that they try to get more information!).
But if the PCs insist on charging ahead, well... let the dice fall where they may.
The caveat: I'll try to avoid a TPK (or allow an immediate rematch) if it's the DM's fault, like forgetting to mention a critical piece of information. This is important to me since I don't put a lot of emphasis on mechanical balance; if the PCs are heading into a situation where they're outmatched, I try to make sure they understand that before it's too late (or at least make sure that they try to get more information!).