The above guide is superb but just to add - The only mission I struggled on to get a gold score was Singapore...After a few failed attempts at this trophy I found this video and went through the mission in 4 or 5 stages and got it straight away (easily!) I played on EASY with the Scar-H and Scar-H SV as my weapons and main gadget should always be the M32 MGL (But can`t access until halfway through mission) Use the video to your advantage as the enemies spawn exactly the same and just replicate the video...As a marker I had a score of 6275 at the very 1st point you enter the tank... Also don`t be scared to reload a checkpoint if you think you could have done a certain section better, I did this numerous times...
Hope this helps
Credit to Whitex Sun for the video...