A great map to go for this achievement on is Shoreline, it has lots of advantages over the other maps, for example;
• The enemies can only attack from 2 different routes, one of which they rarely use and when they do they'll have to approach it from an open area. The second route is the main route they try to use which forces them through a small opening and makes them easy to pick off.
• Both of the routes are very close together which means backing each other up is a breeze.
• Both of the routes being close also means you can place the drone in between them and flag up 95% of the enemies.
• You can see most the enemies from quite a way off so picking them off is easy enough, as added advantage is that when you're low on ammo you can let them get closer before taking them out so you can take turns moving up and swapping out weapons.
Right, the bigger the group the better, only because there is strength in numbers, if you're playing in a group of 4 and 2 of you die the remaining 2 could still pull it off but it'd be considerably harder and much more nerve-racking!
You really want everyone to choose the Auto-Rifleman class and then to choose the M36 SAW, it has the largest ammo capacity in the game and has the added bonus of being able to switch to single shot fire, you need to do this, one or two well placed shots will take an enemy down, it's much better to conserve ammo early on and be selective with your shots then it is to spray and pray. With this in mind you'll be rotating players, if you have enough players try and have 2 players on the left guarding the main entrance and then 2 more players on the right guarding the side entrance and picking off anyone who they can.
When the map starts up all of you will be on a shoreline, move up the immediate hill and there's a building and beside it is the zone that you'll be defending for the next hour, it's marked by 4 smoke flares, you mustn't let any enemies into this zone, if they capture it it's game over!
To the left and in front of the building/zone is a small fence with a gap, this is where most the enemies will attack from, it's very narrow and works well as a bottle neck, it prevents lots of them from flooding in at the same time, go prone beside the building and you'll notice you can see all the way up to the top of a small hill, lots of enemies come down that hill so keep your eyes peeled, have another player go prone beside the tree so that if you miss anyone he can help pick them off, you never want more then 2 people shooting down this route because it's just a waste of bullets, if you have the extra players let them cover you when you need to reload and have them take over from you when you're out of ammo.
From the right side of the building you'll see some wooden walkways, go prone on that and you'll have a fairly big field to cover but you shouldn't encounter many enemies here, you're here just to make sure the few enemies that do come by here don't make it to the target zone, have another player with you just for back up as sometimes they do try and get through in larger groups, but nothing 2 people cant handle, if you have extra players, do the same as I said above, have them cover you when you reload and take over for you when you run out of ammo.
Now, when you run out of ammo the best thing to do is move up to the house on the right side of the main route, when someone tells you it's clear pop round and quickly try and find another gun, the important thing here is to not get greedy, it's better to pick up a gun with 70 bullets then it is to die.
Make sure you place the drone in between these 2 routes and a little bit forward, this way you'll flag up nearly every enemy and make it even easier.
Using this method you shouldn't have any trouble getting the achievement, the enemies are easily visible and can't rush you, just take it slow and don't do anything silly that could get you killed.