Basically you need to be morally good through the game by helping people, listening to specific conversations, refusing to take ammo off others and, bizarrely, playing instruments. This achievement is unlocked at the end of the game, and unfortunately you will only know if you've met the requirements when you are on the last level. There is technically only one missable achievement in the game and this is Enlightened.

You should easily have enough ammo by the time you reach an area called Polis. I will go through this later in the walkthrough. There are 3 achievements linked to the 'military grade' ammo through the game and my advice is to not use any of this ammo until you have accumulated 1000 in your inventory. The game contains two different types of ammo, the easier to find yet weaker 'dirty ammo' and the harder to find but stronger 'military grade' ammo. Once you have selected either Spartan or Survival you can pick normal difficulty unless you want a challenge, as no achievements are based on difficulty in the redux version. Survival is not super challenging, though. Spartan is the easier mode as ammo is more readily available and the enemies don't spot you in stealth as easily. The two that are linked to specific achievements are Spartan and Survival. You have the choice at the start of the game to select your difficulty.

Metro 2033 redux console commands