first complete the game on any difficulty (shouldn't take too long but they do not stack so you have to complete it on every difficulty). once you have done one play through you can choose any level and any chapter, and there's a nice little glitch for all these difficulty achievements. you only have to complete the last part of every level in order. so choose the first level and then the last part, then do the same for the second and so on and then ta da! achievment unlocked. then just repeat the same steps for whatever difficulty your missing.
for people who are struggling with the final boss, there is an easy way to beat him. you need to start off as normal and deactivate the laser. but instead of going into the centre with summers, go back to the higher ground before that. then all you need to do is just move forward so you can only just see summers head above the floor. that way you can just shoot him in the head while he shoots the floor. it may take about 4 or 5 clips to kill him but you probably won't even get hurt. i'll provide a video when i make one, if this descriptions hard to follow just send me a message.