What amaal can i do to meet Imam Mahdi aj. Also can you give me a dua which will be answered guaranteed?

If you were to recite Dua Ahad for 40 days and continue doing so sincerely, you will see the imam of our time InshAllah. More important than that is that he is always a witness for us and seeing our deeds day in day out. When one is watchful over this, you will feel the blessed presence of Imam Mahdi (a.s) and this will give you a deep happiness and satisfaction even more than seeing him in presence. He surely knows how much his shia remember him and yearn to be in his presence so this meeting will be fulfilled some day or the other inshAllah.
All duas are answered because calling Allah has no refusal. The prayer can be delayed or something better can be given, or sins can be wiped away or every thing can be fulfilled including reward for calling upon Allah in this world and the hereafter. Every thing is giving here from the one is is always listening and loves one who calls upon him with sincerity.
Dua of Tawassul is specially for asking for fulfilment of Hajat. Recite that regularly and inshAllah all your prayers will be answered.