Complete the game on Hard.
To do this:
1. I would recomend avoiding melee whenever possible, as your job becomes much easier with less enemies in your way when you're forced to engage in melee.
2. When your armor's shield pops, fall back behind objects and wait for it to restore.
3. When you're low on health, try to execute any Gretchin or Traitor Guard for a quick burst of health. If no gretchin are nearby, fall back and use your bolter to thin the enemy before you engage in melee to heal.
Also: Do not use the chapter select after starting the game on hard. Doing so will cause the game to reset your progress towards beating the game on hard.