DayZ

The best way to do this is to find a friend, go to Server List -> Community -> Filters, type in the name of the Server: “Zavod” and choose ZavodTest (or any similar PvP server with close respawns).
UPS: servers change sometimes. You just need to find a server, where you spawn with a weapon.

You always spawn 10 meters away from everyone else.
You should have few backpacks with a weapon right near your respawn on the ground. Take it, reload it, and go to the main road with a friend.
Then you have to get as far as you can from each other, lay down, press LS to hold breath and shoot.
Me and my friend did it with AKSU, the most common AR/SMG in the game, without any scopes etc.
Take your time, hold your breath, shoot 1 bullet at a time and you’ll get it in no time.
Just make sure that you only see few pixels of your friend as 300 meters is really a lot hehe.

The easiest way to do this is to just boost. Go on a community server or a private server, (preferably a private server, less people).
Then, find a gun, preferably a rifle with a scope, if you can find one.

OPTIONAL: A rangefinder makes this a lot easier, but isn’t necessary. This allows you to see when you’re 300 Meters from someone.

Have your friend stand still 300 meters away while you take shots at him. It works better if you lower their health a little bit already. You can go ahead and get some bandages on other players in for the “Apply Bandages to Survivors 30 Times” achievement while you’re at it.

To the best of my knowledge, you can not get this on community servers anymore. You must do it on a public server either legit or with a partner.

I'm going to break up my solution into categories to help give you an idea of where to go, what to do, and how the game works.

This solution needs 2 people to coordinate

------------NOTES-----------

You don't need a rangefinder for this

I don't believe its possible to do this with a pistol

Get a rifle and find a scope for it. You can TRY to do this using a rifle's iron sights but good luck with that.... Good luck even seeing someone at 300m without a scope.

For some reason, every kind of bullet can be found anywhere and everywhere, but some places (police/guard stations, military camps) almost always have ammo.

If you die, any items you were carrying will disappear with your corpse after a period of time.

-SO-

If you are carrying any items you do not want to lose, and you know for sure you are going to die, either 1: before you die, go someplace you can easily find and drop your gear there. Die. Respawn. Recollect. Or 2: in an emergency, drop your gear where you are.

Your items should remain where you left them even after you leave the server!

The izurvive map (https://dayz.ginfo.gg/) will tell you just about everything you need know about where things are. The only problem is trying to locate where you are in the game.

Food is actually plentiful. There are fruit trees all over the map, they're usually easy to identify and often times have fruit on the ground underneath (except big groves of trees, for some reason a big group of fruit trees rarely drop anything). The fruit will be in 1 of 3 states: raw (hunger + thirst), dried (hunger), and rotten (don't eat, it can poison/sicken you)

Press when walking by fruit trees to make it easier to locate and find food. Again, individual fruit trees seem to have a higher chance of food around them, big groups of fruit trees don't at all for some reason.

Water pumps: if you have no bottle, it is safe to drink from the pump as long as you're wearing gloves. Hand wraps work (crafted from rags). No gloves = risk of getting sick.

------------FIND A RIFLE AND A COMPATIBLE SCOPE-----------

A common rifle you'll come across scavenging in homes is the Sporter 22.
It uses .22 LR bullets. These are also somewhat common
It uses a clip! These are somewhat common too.

But finding a hunting scope for it is near impossible.

This is the ideal scope too. It has x12 magnification

Police/guard stations: good for finding ammo for almost everything. Bad for finding rifles or scopes.

If you can't find that hunting scope, you'll have to raid military tents/camps/miliary conexes(the camo colored cargo crates) to find a different rifle and a scope to go with it.

You will likely never find a scope and a rifle in the same place. Its not too hard to find at least 1 of the 2 at any military location.

Not all scopes are the same either. Some scopes offer no magnification at all. If the scope doesn't have a X magnifier on it, dump it!

Once you find a rifle and a scope, here's what I suggest you do next!

----------THE PERFECT PLACE FOR THE 300M KILL----------

I went to Krasno airfield. On the izurvive map (https://dayz.ginfo.gg/) its at coordinates 12100, 12550.

The medic field tents at Krasno always have tetracycline packages and bandages!

There is also a trail leading to a military barracks nearby (location: 10950, 13400). This is where I found the rifle I used to get the achievement. (be aware: wolves tend to spawn around there. If you hear howling, RUN. You will be safe if you can jump on top of something or into a building and closing the door.)

For me, I found the rifle I used to get this achievement at those barracks. I found the scope in a different camp however...

Anyway, Krasno airfield is flat and open. It also has features you can use that can be used to put your partner 300 meters away from you for this achievement.

On the Izurvive dayz map, look at Krasno airfield. You will see yellow balls lining the side of the runway. In the game, the yellow balls correspond with lights on the sides of the airfield.

On the map, looking at the top of the airfield, you will see roads going north and it looks like an empty grey triangle with the runway as its bottom. The yellow lights lining the bottom of this triangle: if 1 person stands at one end, and the other stands at the other, you will be standing roughly 320 meters apart. If one person moves closer to the other by 1 light, you will be roughly 300 meters apart.

----------THE SET UP------------

You and your partner don't need to meet up at the same time. You can get to Krasno airfield and quit the game. When your friend gets there, you can spawn in exactly where you quit when you reload the game AS LONG AS YOU REJOIN THE SAME SERVER.

If you quit and join another server, the game will place you (with all your gear!) at a random spawn point.

When you and your buddy are ready, before moving into position:

the guy getting shot: I would recommend removing all your clothes and items except your shoes (otherwise your feet will bleed as you move to position)

the guy doing the shooting: using your bare hands, punch the guy you're shooting until his health is yellow/red

--------THE SHOT----------

You and your buddy are in position

You have a rifle

You have a scope

To have a steadier aim, hold down the button to lay flat. This will help a lot.

Pull out the rifle, and aim it and..... there's no scope? I put it on! What the hell?

This game is really stupid

Press on the d-pad to look into the scope.

To temporarily steady your aim further, press and hold to suck and hold your breath.

If you have a damaged rifle/magazine or the game just wants to be a dick and the rifle jams after shooting and you need to shoot again: hold down to unjam it.

I would recommend aiming center mass unless you have a really good scope. Aiming is hard to control. Dayz doesn't offer a fine, precise control when aiming down a scope

-------GOOD LUCK--------

I had a SK 59/66. That rifle's scope is about 4x. Originally I did this, I shot my partner who was at full health. The shot did not kill and made them bleed. So be aware of that. I'm not sure if shooting someone who dies from bleeding out counts.

The 2nd time I tried, I got a kill on hit and unlocked the achievement.