According to Buddhism , who achieves Nirvana? All phenomenon are nonself then who achieves the Nirvana?
Your question is invalid. If all phenomena are non-self you cannot ask "what self achieves the phenomenon called Nibbana".

Prince Siddharta attained  Nibbana and became the Buddha. But that is a statement in conventional reality. When you say "all phenomena are non-self", it is a statement in ultimate reality. Do not  try to mix up the 2 paradigms. Either you speak in ultimate reality or conventional reality. Pick one at a time.