The hard achievements stacks with the casual and normal difficulty achievements, so if you beat the game on hard, you will obtain all 3 difficulty achievements at once at the end. You can replay all chapter via timeline / chapter select to obtain the hard achievement, is not needed to start a complete new playthrough. To obtain the hard achievement via timeline, you have to replay ALL chapter
(chapter 1 - 5) AND as last chapter 5-3 to get your achievement.
Hard is not really hard in this game except the final boss fight. Some users have really problems here, so I have create a short video and write down my strategy. Honestly you need only the time rush + melee skill and some pistol bullets to kill the boss easily ;)
First of all, you can not damage the boss directly. First you have to clear the enemies in this area and avoid the bombs from the boss.
My strategy uses only time rush and melee skill (Hold LB + B near an enemy). Melee works only in combination with time rush and kills enemies instantly with one hit. After the cutscene run down the ramp with time rush and melee the first enemy in front of you. Get in cover and let time rush refill to 25% it can be used again. Use time vision to detect the remaining enemies and kill them all using time rush and melee. The most important thing is to avoid the red bombs on the ground from the boss. If you stay longer on a position he will throw the bombs and you will die instantly.
The best way to avoid bombs thrown at you is using time dodge (push LB short)
After you kill the first 3-4 enemies the boss switches the position and throws a giant bomb on the ground, run away as far as possible! After the bomb detonates go back and the boss becomes vulnerable, shoot him with your pistol, one shot is enough.
That was the first of two rounds. The second round starts now and more enemies arrive at the swimming pool. Once again use time rush and melee to kill all of them and the boss will start to use the giant bomb but this times he throws 3 bombs instead of one! So you have to run from one end to the other and back to avoid the explosions. If you have managed to avoid all 3 bombs you can shoot the boss again one time and the fight is over.