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.
And suppose someone were to ask you, 'This fire that has gone out in front of you, in which direction from here has it gone? East? West? North? Or south?' Thus asked, how would you reply?
I suppose I can read your question in two ways:
The Buddha (or arhat) is like the fire, dependent on causes and conditions. When it has gone out, what has gone out?
What does the Buddha (or arhat) depend upon, that has been "uprooted"?
I think the answer to 2 is form feeling sensation, etc., and the answer to 1, what is put out when nirvana or final nirvana is reached, is conditioned existence, or maybe suffering, or dependent phenomena, or ignorance. It probably depends on your framework.