Given the karachi weather these days..lawns are preferred all year round generally (apart from rainy days which are few and far in between)
Apart from Subzazar on Shahrah-e-Faisal, there are Lalazar lawns as well as Kashmir Lawns on Kashmir Road.
There are some pretty decent lawns/halls in North Nazmabad as well on both sides of the road from Sakhi Hassan till KDA Chorangi
...and if the cash outlays permits, try and get hold of Marriott or PC's Poolside :~)
Good luck