first of all dont expect mehrans ac like the ac of other cars like toyota , swift or honda.
at nights its cool but at day time when its hot, it just manages to make you feel better. Just make sure to do folloing:
check if the outer air intake is closed properly and not letting hot air inside the car.
check for the heater setting that it should be closed and not mixing hot air with cool air. it can be checked near the air blower.
check and make sure to close air coming from door rubbers....
otherwise, get it checked with the ac wala if it has proper gas filled or not.
at peak time, the ac of mehran cool air flow remains equivalent to 24-26 degree and at night it can goes to 12-14 degree.