At level 15 I seem to do ok against most things but then I see another character wander up and smash things in one or two hits. Obviously their damage output is significantly higher than mine.
What is a decent damage output for a level 15 character?
You must be doing something wrong. Right now I'm 14 in a 17-18 zone doing fine. Make sure your gear is up to date. Your putting your points into the correct attributes you want. And you are getting skills that you will use
Not really enough information to judge. It will vary depending on what your weapon is, what your class is, what skills you have on your bar, what attributes you chose, and finally what it is you're fighting.
Not to mention it's likely as above that it's just someone high level stomping stuff (I'm typically above level for the content I'm doing because I explore so much).