I'd actually forgotten that encounter. It wasn't that bad for the group I ran the adventure for, because as I recall, the Roper was all by it's lonesome. Now the dragon fight, hoo boy, was that a hot mess...
This does bring up a big pet peeve, monsters I truly despise. "Gotcha" monsters. Things that are either perfect at pretending to be other things, or things that have special abilities that there is no way to avoid without knowing what you're up against in advance.
Oh look, it's a little bunny rabbit on a tree stump...nope, sorry, it's a horrible monster! These things are so prevalent in D&D that any experienced player is suspicious of everything. I still remember a few months in a row, where Dungeon magazine had adventures featuring beautiful maidens who weren't as innocent as they appeared. I didn't think much of it until I ran an adventure and I said "a lovely nymph, a triumph of nature, steps out into the path ahead of you, and pleads with you for help", and a player said "I shoot her with an arrow."
"Why would you do that?!"
"She's probably a succubus or something."
I was flabbergasted (she was a Hag who had been polymorphed into a Nymph) and struck speechless. "You don't know that!"
"Are you telling me she's not evil?"
I walked away from that session having learned a good lesson. Fool me once, shame on me, fool me twice, shame on you...