We'll nee some more information before offering suggestions.
1. What's the current condition of the AC system? Are the compressor, condenser, hoses, evaporator, etc in good shape? Is the system charged properly?
2. Have you taken steps to minimize outside heat from entering the cabin? Are all engine compartment grommets and seals in place? Are the footwell fresh air and heater doors closing properly? Is the heater control water valve closing properly?