Red Dead Redemption

This solution requires the Undead Nightmare DLC, but it`s the easiest method by far.
Once you`re able to go back to Mexico, head to El Presidio. Climb the ladders and get on one of the cannons (I used the ones at the southern gate). After you`ve done this, just keep blasting the zombies. I got this achievement in about 30 minutes doing it this way.

NB, if you keep moving the cannon all the way to the left or right and wait until you hear the zombies moaning, you`ll find a group of four or five have spawned making your grind a little easier. Hope this helps :D

Ok just got this today. FYI its just a pain to do but you can do it solo. I'd recommend doing this with a boost buddy to make it easier.

I did this when I got the dynamite unlocked, think it was around lvl 13-15. dynamite makes it easier to get the law after you but there is a cannon as well you can use.

Start a private free roam session, and travel to El Presido. If you have dynamite throw it at the main building and as soon as it explodes you will have the law after you. alternatively if you don't have dynamite, get on the cannon above the transport wheel and fire it at the tower to the right of the cannon.

now mount the gun near the transport wheel and mow down enemies as they keep coming. if you got a buddy have him cover the back entrance as they spawn randomly on the front and back paths. if you are doing it solo just make sure to pick off enemies at the back door occasionally with your repeater to get them to respawn and hopefully come down the Gatling gun side.

that is pretty much it, just keep killing as they come, you will get good xp if you can keep a high wanted chain kill spree and get some leveling done as well for the level 50 achievement. also if you are solo you will probably die a bunch, just repeat the initial steps to get the law spawning and keep grinding out the kills.

notes:
- you can keep track of your kills under the pause menu > stats > combat > "People killed with the Gatling gun"
- for some reason i tried mounting the other Gatling gun way up top in the tall tower but the law wouldn't keep spawning. I didn't try with a partner who was on the ground level, so the law may keep spawning if one of you is still near the bottom Gatling gun
- note from crewdy4923: "make sure you click to exit multiplayer if you want to continue getting kills later". (if you don't I guess stats don't save unless you do something where you see the "saving" prompt in the corner)
- note from STLfan2468: SP and MP kills do not accumulate on one score. They are separate and you must either obtain 500 kills in SP or 500 kills in MP for the cheevo to unlock. ie. 250 kills in SP and 250 in MP will not work.

Ok just got this today. FYI its just a pain to do but you can do it solo. I'd recommend doing this with a boost buddy to make it easier.

I did this when I got the dynamite unlocked, think it was around lvl 13-15. dynamite makes it easier to get the law after you but there is a cannon as well you can use.

Start a private free roam session, and travel to El Presido. If you have dynamite throw it at the main building and as soon as it explodes you will have the law after you. alternatively if you don't have dynamite, get on the cannon above the transport wheel and fire it at the tower to the right of the cannon.

now mount the gun near the transport wheel and mow down enemies as they keep coming. if you got a buddy have him cover the back entrance as they spawn randomly on the front and back paths. if you are doing it solo just make sure to pick off enemies at the back door occasionally with your repeater to get them to respawn and hopefully come down the Gatling gun side.

that is pretty much it, just keep killing as they come, you will get good xp if you can keep a high wanted chain kill spree and get some leveling done as well for the level 50 achievement. also if you are solo you will probably die a bunch, just repeat the initial steps to get the law spawning and keep grinding out the kills.

notes:
- you can keep track of your kills under the pause menu > stats > combat > "People killed with the Gatling gun"
- for some reason i tried mounting the other Gatling gun way up top in the tall tower but the law wouldn't keep spawning. I didn't try with a partner who was on the ground level, so the law may keep spawning if one of you is still near the bottom Gatling gun
- note from crewdy4923: "make sure you click to exit multiplayer if you want to continue getting kills later". (if you don't I guess stats don't save unless you do something where you see the "saving" prompt in the corner)
- note from STLfan2468: SP and MP kills do not accumulate on one score. They are separate and you must either obtain 500 kills in SP or 500 kills in MP for the cheevo to unlock. ie. 250 kills in SP and 250 in MP will not work.

If you complete all story missions there is a defensable browning gun in El Matadero. You only need to kill 500 people (townspeople and lawmen), its a lot easier cause you have already racked a bunch through various story missions.

You may be tempted to do this in single player since you get a lot of mounted kills in missions but do it in multiplayer. It's a little more tedious but you get XP with every kill which makes the extra work worth it.

I did this at El Presidio. If you don't want to be bothered by other players do it in a private session. Go to the entrance that has the machine gun and climb up to the cannon on the other side of the doors. If there is a soldier inside the compound kill him to get a wanted level. If there isn't shoot a box of TNT or a lantern to get a bounty. Now hop on the cannon and start lighting up soldiers as the ride up the road.

It only happened a few times but be wary of soldiers coming in the entrance behind you, as it can be annoying to get domed from behind. All in all if you use this method in multiplayer you'll walk away with your achievement, at least 25,000 XP, and at least a few MP challenges.

NOTE: This guide is a useful one if you have not got the Undead Nightmare pack - If you do see the top guide for a quicker method.

I had tried with the above guides on numerous occasions, i was getting kills but was often unlucky with spawns behind me and the army not being able to be shot.

I tried on a different gun within the fort, this being the cannon just above the transport spot to the right of the gate as you head towards the southern side of El presidio.

On this gun i was able to achieve roughly 100 kills with little or no effort.

It is important to note the spawns:

1. you will have army from the direct left which will be at the top of the hill

2. Men from the distance heading towards you on the road (easy when they get of their horses to just blast away)

3. On the right on the hill. The hill on the right can often be a bit glitchy and the men will stand on the side wall of El presidio and not be able to shoot you and vise versa. This is however beneficial as it saves you from being overwhelmed. Sometimes they will fall off the cliff edge and might survive so be wary of any shots from here.

I managed to grind out these kills in about an hour.

If you don't mind doing this in a longish time then this is the guide for you. Buy doing coop missions, you will not only work towards this achievement but will also receive a mass of xp points which is the quickest way to level up. Simply, keep going through the escape mission. Here every time you have a chance to get at least fifty kills with cannons placed at the ridge adjacent to the church, but also earn a mass of xp for completing the mission which generally takes around 12-18mins.

And again- this guide is not for boosters who don't enjoy the game whom just want the achievement.

This took me a while, but if you've downloaded the Undead Nightmare DLC, you can get this on there as well.

Once Mexico is unlocked, travel to El Presido. Use the cannon right next to the front gate, and go to town on the undead. They spawn very quickly and in large numbers. You can get this from scratch in about 5-10 minutes. Plus, you will never get attacked from behind.

When you shoot quickly and there are none there, wait about 10 seconds and another wave will spawn.

There is plenty of ways to do this all of them at El Presidio, the simplest and easiest is on the Undead Nightmare DLC.
All you have to do is get on the cannon that sits on a small tower next to the main gate and scan the road going into the fort. Simply blast any so zombies you see, remembering to move the aim to the cannons limit of it's horizontal aim this refreshs the road of zombies. With this you should be done in no time.

Theres two other ways, both exactly the same but one done in either the main single player game or in online free roam. I would recommend doing it online as it also helps you rank up in order to get the achivement for reaching the maximum level. It also has the added advantage of allowing you to bring a friend along to help.

All you have to do is draw attention to your self by killing someone or throwing a stick of dynamite and then clearing out the fort of any lawmen.Then just get on the gattaling gun on side of the main gate and just cut up any approaching Mexican soldiers. Be sure to press the left or right buttons on the D-Pad to get a better camera angle as some enemy's can hide in areas that can only be hit at certain angles.
Some soldiers will attack you from the side gate, if you have friend helping you he can keep an eye on that side door allowing you to stay on the gun. If not you'll have to keep an eye on the radar and when soldiers come round your side you can deal with them before they become too much trouble.

Without a friends help you are likely to die a few times but it's easy enough to get back on the horse.
It's also worth noting that it's 500 kills needed in one single game mode. So if you do it in free roam you need 500 kills regardless of how many you get during single player. However the kills can be shared across the the three types of mounted gun.

Thought I'd make a video about this, to get this very easily you need undead nightmare dlc, once you are about 3/4 of the way through the story line you will be able to enter mexico, in mexico go to El Persidio (I believe that's how it's spelled) and just like in the video climb the ladder and start shooting zombies, just look to your left and back again and zombies should respawn.

If people start showing up and they have no heads that means the game has glitched out and the kills you make from that point do not count, if this happens to you just kill yourself and press Y to restart by nearest town.This shouldn't take long, i had 0 kills took me around 20-25 minutes to get to 500.