know-it-all
noun
°One who makes it appear as if they're an expert on a subject while there are also signals (subtly or blatantly noticeable) that they don't know very much on the subject or at least as much as they're acting like they do on the subject; pretending to knowledge on a subject matter -- normally intended as a pejorative.
"We had it all figured out, but this know-it-all marched in with "the correct way of solving it", leaving our experiment in shambles."