You can check following things which are Add extra load on your engine thus making it generate more heat:
1- Alignment/Cambering of front tyres
2- Tight Clutch Wire/Tight Tappets
3- Compressor Gas Pressure
If all the above are ok you should consider the following:
1- Genuine Coolant. Caltex coolant is superb though expensive.
2- Making the Radiator fan direct. (Although not recommended but somehow it works as long as you want AC performance).
3- Adding Extra fan (With relay for full current) infrornt of condensor.
4- Large radiator of more capacity such as "3-Nali radiator".
5- Check the condensor. Mehran's condensor is notorious for acquiring bends during water pressure applied while car being serviced. you can go for buying another one of some good company which is heavier and tougher.
6- Think about fan with more blades than a normal one. Mehran comes with 4-blade fan. Buy the one with 6 blades.
Basically you need two things to make your AC perform well:
1- Engine Cooling
2- Optimising Load on engine