Full credit to "Blaze Naruto Shippuden" from PST for this tip.
Easy way to get credit for master difficulty. I personally felt this game is unfair on master difficulty. The enemy goes down quickly but takes you down even more quicker. A grunt killing me with a pistol in two shots from 30m distance is just insane. Titans were surprisingly easier to kill.
1. Start the game on master difficulty. First chapter doesn't matter
2. Get killed intentionally. The screen greys out with X to respawn. Do not press x but press start and change difficulty to easy. Do not press x still and the game will reload itself from last checkpoint
3. Play through the rest of the level and complete on easy but get credit for master difficulty completion.
4. If you somehow died on easy after switching, do the same but this time change to master and repeat the steps. Death by fall doesn't matter as it doesn't prompt respawn. Only death by gunfire and explosions count as death.
5. Need to do this for every chapter and can't be done at one stretch. Eg. Finish first chapter , let the next chapter load. Quit to main menu, level select chapter 2 to start on master and so on.
6. Chapter 4,5 and 6 has three sections each. For these you will get credit for only first section initially. The other two sections need to be played on easy to enable chapter and section selection. Once enabled replay the last two sections using same trick from steps 1 thru 3.