No, you aren't. That person was wrong, and frankly, could be reported for that kind of behavior (harassment). People are not expected to read guides for casual content (and most content in the game, honestly), and a lot of content you have to mess up A LOT to die or have your entire party, healer, etc all messing up. There are very few things that one-shot kill you outright in this game, and dying in a dungeon isn't a "waste of time", it takes all of 40 seconds to run back and hit the boss again and kill it. Anyone who isn't up to the possibility of a silly wipe shouldn't be quing for dungeons.
You're fine, and keep being awesome. You'll get the hang of it, and most people in this game aren't jerks like that. I take it the other two people in the party weren't supporting/agreeing with that person (if I was in that party, I would have initiated a vote to kick them for harassment if they wouldn't quit after being asked nicely, personally...)