This has been well documented on other sites and I wanted to post here as well in case anyone came here looking for help.
The Survival game mode is unlocked at level 15, however you can accept an invite to one at any level and play it just fine.
The best/easiest level to complete all 10 waves is Boneyard, there is a portacabin on one side that sits very close to the wall.
Taking cover behind there will ensure you rarely get hit by anything. The biggest issue I found are the helicopters but taking out the gunners either side should be the priority with them, then they will just fly around and not do be able to shoot so you can ignore them after that.
Also the minigun is good for taking them down pretty quickly too.
This method can be done solo, but is quicker with a group.
Main point is to stay behind the cabin and only come out for ammo/armour in between rounds, the enemies WILL come to you eventually.
Any tips or suggestions please put in the comments