Arguably hardest achievement in the game that will take you many hours of trying to get it. Best to look up for a video on youtube (which I did).
Take Fetisov, two girls and grandpa on this. Fetisov can take upper machinegun (for about 12 first waves) while grandpa can have one down right. Two girls can run faster than others and that is crucial. Buy only a gun for one and AVS rifle for the other. For the rest of your credits, buy mines mines mines. And a few grenades :D
For the first part of the mission, make sure you get every boxes (the one in the building down left on map contains heavy machinegun which you will give to Fetisov).
Everything here is about micro management and you will do most of action with girls. First few waves are easy after which you need to start placing those mines on proper locations where enemy waves will spoon. (best to watch a video). Also you can wait with a grenade for enemy to spoon. Make sure to release a grenade around 3-4 seconds before they spoon. That way they will be blown away immediately. With mines and grenades you will do most of the work. After 12th wave move Fetisov onto down left corner to repel enemy waves with machine gun.