In Cars like Mehran & FX you can run AC with very efficient cooling Given that you take the following steps:
1- Genuine Coolant (Like that of Caltex, RED in Color). I have bought one Litre Bottle from a Caltex Petrol Pump for 480/- recently.
2- Hi-speed Radiator Fan with Six blades (In Mehran the factory fitted fan consists of four blades). For that, you will have to search Kabari Market like Bilal Gunj in Lahore for Genuine six blade Fan. While Installing it, connecting a seperate Relay is recommended in order to provide full current to get full rotation Speed. And yes, do not forget to make it Direct as is the custom. 
3- Get your Clutch wire loosened a bit so that whenever you want to shift the Gear, the clutch has to be depressed with full force. (It will compromise comfort, since applying full force means your leg in under stress). Accordingly, the tappet adjustment should be done by loosening them which makes the engine sound a bit strange but that will reduce the load on your engine. I know this idea may sound rather ridiculous but believe me it works well when you want to make your AC fully functional. Reason is that as soon as you turn on the AC, the compressor puts load on the engine. About setting of clutch and tappets, you should consult a reliable mechanic who knows this idea works.
4- Installation of an "Extra Fan" for AC Condenser to increase cooling. Again, this kind of fan is not common and have to be searched from Kabari markets. In Mehran, there is a small space for extra fan installation so again, a mechanic who DOES KNOW about all this should be consulted. This fan is installed in such a way that it starts ONLY when you turn the AC on. (Also make sure that the condensor is clean and free of dust particles which accumulate due to pollution).
5- Get your Car's Compressor gas checked. It requires that the volume and Pressure of the gas are sufficient and according to factory fitted compressors' parameters. Normally, they fill Chinese or the so-called "12-number" gas which is not good for piston compressors (Mehran's default compressor). So get it filled with Genuine R134a gas.
6- As you all know already, the temperature valve that controls the flow of liquid in the cooling system of car has to be called off from its duty. This makes the flow of liquid un-interrupted no matter what temperature your engine achieves. The Valve should be greased well in order to avoid rust and placed in a safe place during summers. and please, Don't forget to install it again during winters otherwise....!!!!
7- And the most important of all, the Radiator of course. Installing a "Three Nali" radiator makes cooling significantly better since the water flow and dissipation increases. But DO NOT go for desi OR local made radiators since there metal is in-efficient to do the job. A custom-made radiator, Cut-out from a bigger genuine radiator is recommended. For that, again you will go to kabari market to find some Genuine bigger radiator in order to modify it. Give it to a "Radiator wala" so that he could cut and weld it to make a genuine "Three Nali" radiator.