"I didn't think that hydras were so difficult to sneak upon!"
I'll grant you that I try to be as clear as possible in my descriptions so as to avoid misunderstandings...
but... if someone has never faced a hydra before, how would they know how difficult it is?
Generally I'll confine myself to things like, "You realize it has several heads, right? It has more "range" than most things in hearing, sight and smell because it has so many eyes, noses and ears. And it doesn't appear to be just some mindless lump, it looks pretty alert. Still wanna go for it?"
In short, I try to avoid giving players a
precise look into the mechanics of what they're about to try, but if I say something like the above it should be obvious I'm trying to say, "I think this is
a lot more difficult than you think it is. Don't forget that I'm the GM, I'm the one who
has to decide how difficult it is."