Hello Rin Power,
Welcome to the forum.
To use an English idiom correctly, will probably depend on where the other person is from.
If they use British English (BrE), then to me, "Oh, dude, you know your job", does not work. It may, however, work in American English, or in any of the equally valid English variations from around the world. Members from those regions, may be able to provide you with an idiom which works for them.
If the person you are addressing is a BrE speaker, and you know them well, the first colloquial thing that popped into my head was:
"Well done mate! You certainly know your onions." http://www.usingenglish.com/reference/idioms/know+your+onions.html
That is very informal, and I have pitched the tone to match the very friendly one you used. It probably sounds bizarre as a phrase, but I think it's quite commonly used. It simply means that they are an expert in what they do, with a great deal of knowledge.
A more formal way of saying it would be to replace, "Well done mate!", in my example, with "Congratulations!" or just "Well done!"
Does that help?
Student or Learner