How do you reduce the discharge temp
on a low temp condenssor?
By lowering the suction gas temperature.
If you seek to reduce discharge temperature on a system with normal/acceptable return gas temperatures (such as would exist on a low temp R-22 system, there are liquid injection valves avialable for that application.
These valves are connected to the liquid line and meter in small amounts into the suction line just upstream of the compressor's inlet in response to discharge temperature.
You need to know what temp you want to limit the discharge temperature to, and all the regular info about your specific system and compressor.
Go to your local wholesaler and they should be able to help you.