Lenovo — Defective phone sold to me. Refusing to replace me another piece and harsh attitude by service center

Address:

Guntur, Andhra Pradesh, 522002

Bought Lenovo A6600 plus in a retail shop, "Options Mobiles Pvt. Ltd" for Rs.8000 on 1st April 2017.
It is taking 5 hours to completely charge this phone and full battery is completely discharging in just 3 hours with normal use (no games).
When I took it back it to shop on 12th April for this problem, Lenovo promoter in that shop, Mr. Bharath, who sold this piece to me responded very rude this time and denied for replacement.
Mr. Bharath said "though you came within 2 weeks, I wont replace your phone. You should go to service center and get it repaired". He even gave a wrong address to the service center.
Finally I have traced out the address to service center and gave my phone at service center on 13th April, 2017. Next day, they have said that it is Mother board problem (so this is definitely a defective piece, confirmed by Lenovo service center). Lenovo service center employees said, it will take another 10 days to replace the mother board.
It has been more than 16 days now. In addition to this delay, the Lenovo service center employees are answering very harshly each and every time when asked for the reason for delay. Still the service center employees are saying that the mother board is not yet shipped to them and it will take atleast another 10 days to fix it.
Please resolve this issue, I dont want a defective piece. Because everytime I had an issue like this I cant give my phone to service center and wait for weeks together. I am a doctor and lot of my works are getting delayed just because I dont have a phone now.
Hence, my desirable resolution of this is to either replace the defective phone sold to me with a new phone or refund my money Rs.8000 completely.