You will not always die of one bullet. For now, it may be a glitch, but I can name numerous times where I survived, even if I was killed shortly after, and you will know if it happens.

You will definitely survive multiple shots when in a vehicle.
There are just too many things to worry about in vehicles, so they have obviously given you some chances there.

You will more often survive an extra shot when facing a tough enemy, such us an immortal.
These guys got me good quite a lot, and I'm not yet sure if it's a glitch, or Treyarch trying to add some favour to the player's side, but you can and often will survive one bullet. This however does not mean you should get shot, because you can't count on always surviving.

In the Zombie section of Demon Within, you will often survive 2 hits, but you really need to be very careful as it gets harder, because even they can one hit you, and it is extremely difficult.

This as I said may be a glitch and may be patched, but for me it will not matter. I beat this achievement within 15 hours of my full playthrough (I was often too annoyed and went over to zombies or multiplayer)

You are going to die. A lot.

Due to the game being optimised for multiplayer (cool for some, not for solo) it is much more open, and as such, hiding places can be hard to find. You are bound to cop an odd bullet at the most unfair of times.

There are plenty of very challenging and frustrating sections. I can not stress enough how important patience is.

Take your ♥♥♥♥♥♥♥ time. Do not ever attempt this if you are in a hurry, you will become infuriated.

Upgrade your martial and chaos cores. They are extremely helpful. Active Camo will play a huge part in your playthrough. Always use your cores when you have a chance, you can disable numerous enemies, or even sneak right past them. Don't rely on control cores because the robots are not as helpful as you may think.

Rely on your team. I was amazed at how helpful the AI is. As long as you move forward, they will slaughter the entire field for you, however that doesn't mean you shouldn't kill when you get the chance.

Remain Calm and assess everything.
If you find yourself dying a lot, try lots of different methods, because there are many different ways to approach the levels.

Always be behind cover. If you're in the open, you're dead. If you move from cover, go straight to more cover. Until you're sure there are no enemies, never leave cover. Unless you have something dropping grenades or missles on you.

Speaking of grenades. In your class section you can add two abilities. One is up to you, but as soon as you can, get the trophy system ability the pushes away explosives, and upgrade it to push away rockets. It WILL save your life. This game isn't world at war, but it will still catch you off guard. As for the second slot, as cool as it is, wallrunning and boosting will not help you often in Realistic. There is one section in Vengeance where it helped me skip half the level, but you do not need it until then. Running accross walls and flying in the open will get you killed.

Always have your DMI view on. There is an option (the 3 button for me) which will make taking out enemies much easier, as you will be able to see their heat signatures whenever your team is looking at them. It will also help you calculate grenade throws. Very useful when you're moving and need to be sure there are no threates.

Use your active camo. As I mentioned earlier, this ability is by far your biggest asset. You can move from one spot to another without a single enemy catching on. It is also useful for getting out in the open and spotting every enemy. In vengeance, if you have the boost and wallrun ability along with camo, you will be able to breeze through the 2 large stealth sections and only have to stop to recharge camo (which will also net you 2 accolades if you remain unseen). That being said, against bosses (Mothership) and Robots, it is not as useful, so be mindful of that.

Be ready to put yourself in harms way to complete an objective. If you're trying to blow up the core in Hypocenter, (this is a good example as I was often having trouble here) throw emp grenades and activate your speed boost core, because as soon as you reach certain objectives, a pre rendered animation will take place, landing you a checkpoint. However, be sure you have the seconds to make it, otherwise you will have to start from the last checkpoint.

Play through once on easy. I did Realistic on my first playthrough, and I suffered for it, because I had to level up as I went. If you can unlock everything before your playthough it will likely help you immensely.

Final tip. Double Tap. Your enemies are not playing realistic, they will often survive a couple of bullets. Always be sure you've killed an enemy before moving on.

Never run into the open while enemies are alive. Instant death.

Never put yourself in harms way if it has been a while since your last checkpoint. You don't want to have to do big sections all over again.

Never stick to one thing. If you have trouble, don't keep trying a failing strategy. The maps are huge, so go a different way, or try different cores.

Never stay out long when using cores. Only use cores for a split second as even that may be too long.

Don't move until your cores have reset. You may find yourself relying on them and not having them.

Never underestimate the bigger enemies. Boss fights are awful, they will be the source of most of your anger.

Other than all the time, you will find the game becomes much more difficult at the half way point. As far as I'm aware there are a maximum of 4 times where you will face infinitely respawning enemies. These are the core battle in Hypocenter (If I'm not mistaken on which level that is), the zombie battle in Demon Within, the area where you must drill spikes into the ground in Rise and Fall, and the air battles in Sand Castle. I only had trouble with the zombie and core battle, but these are worth noting. I apologise if I have the wrong level name/order of encounters.

During the zombie fight, it is advised to have camo, as it will give you enough time to reload and prepare. Zombies will not move if you are invisible. Use the ammo crate to reload if possible depending on your weapon. I played with an LMG so it was much faster for me.

During the Core fight, always be ready for an odd robot. There are robots both running and shooting at you. Remain behind cover and listen, because if one runs at you, you wont have a chance to kill it if it gets too close. They are instant killers, unless they get you from behind, not that you will have long to react though if you do survive.

As for other parts, take your time, and rely on camo. You can often run straight past an enemy and they wont notice you at all as long as camo is active. You can use this to approach objectives and gain a checkpoint if you assess situations well.

If you are facing a boss or tough enemy, remain in cover and send the fireflies. The fireflies will distract enemies for a long time while you unload on their head. When you hear the beautiful pop of an immortals skull exploading, then you can rest easy.

The hardest moment for me was actually the mothership fight in Lotus Towers. You will only survive a few bullets if it fires at you, but you cannot remain still because it also fires rockets, which were often unnavoidable for me, even when behind or under cover. Do not use the rocket launcher. It will only take off half the engine at a time, meaning you will need to put yourself at risk 8 times to win (I believe that is the case). Use the minigun in the center armory. Destroy the engines as fast as you can. Each engine has two fazes of life, one to disable, and one to destroy. The minigun will destroy them extremely fast, and will allow you to chip away lots of health from all engines before the mothership runs away to fire rollers at you. When they show up, stay in cover and take all of them out. Be aware that the ship will come back, but if you don't take out all 8 rollers, they will sneak up on you. You should be able to destroy two engines and disable two more with the first round of a minigun, but there are no checkpoints in the fight, so you need to be super careful when trying to destroy the others. If it launches rockets, run and pray, don't worry about bullets while you run, because you will likely survive and make it to cover. You will not survive the rockets.

You wont have fun all the time on this challenge, but it is doable. You really need to be patient, and always look as far ahead as you can for enemies. Always rely on your cores, and always keep your gun loaded.

Good luck everyone.