This isn't easy, but it's doable. First off, you need to survive all waves of zombies and actually win. You need to survive, and so does the technician. If you die, even though you come back as long as other players are still alive, it won't count. Don't die once.
Try to use as little ammunition as possible over the first 3 waves. Don't shoot any vanilla zombies, save your ammo only for bloaters, ferals, and juggernauts. You'll need all your ammo after the 3rd wave when things get crazy. I have found that the Cleo axe uses the least amount of stamina for the most damage. You get way more swings with the axe than with the heavier weapons, and you'll need those extra swings.
Late in the game, pick your shots, fire in controlled bursts (unless it's a juggernaut) - it's all about conserving ammo right to the end.
Stay outside the wall to deal with the zombies. Never shoot at them from the inside, that's a major waste of ammo. Try to spend as much of the time as possible outside the walls fighting them off, even late in the game.
If possible, coordinate which gun each member will use, try to vary it as much as possible, so that there is enough ammo to go around. If everyone uses guns that take Cleo ammo, you'll find yourself running short pretty quick.
At the end of each round after the 2nd, you'll start getting Cleo drops. If you don't empty them out, they'll eventually self-destruct, so you want to focus on picking up the most important things. Focus on picking up healing supplies, walls repair kits, and ammo for the gun you're using, leave everything else, and let the other players grab the rest. Enough Cleo drops fall between each wave that each member should be able to stay well supplied if you've coordinated well.
Any time a wall gets to the "yellow" status, repair it. You should have enough wall repair kits from the Cleo drops to take care of it. You can carry 6 in one slot, so make sure you grab them when you can (for some reason I have noticed that sometimes they won't stack correctly and you'll end up filling up an empty spot with a wall repair kit when it should stack with your others, so beware). You don't want a wall to come down, that increases the chances that the technician will die, and is too hectic to control well. It will probably happen, and if it does, everyone should fall back to the technician while one player repairs the broken wall, and another covers them until the wall is fixed.
Use your mic. If you're running out of health and don't have any healing supplies, have someone drop some for you.
If you're late in the game, and running out of healing supplies, stand on the top of the wall and shoot zombies to thin them out for your buddies until the end of the round, or until someone can drop you healing supplies. Or if you have the standard shotgun, firing through the wall openings from inside the base at zombies that pile up on the walls also works. Focus on staying safe until you can get healed, then get back out there.
Use the explosives, placeable gas canisters, or whatever other explosive devices you have. Set them in the corners of the walls on the outside, so if you get a zombie pileup in that area, you can detonate it to wipe them out. Set a gas canister next to the technician. If he gets mobbed, you can shoot it and wipe out everything. Also, drop the automated mine layers around the inside of the base to thin the herd if zombies break down the walls at any point. Obviously you can only bring one in to the game, so coordinate with your buddies which explosives you'll bring in.
Always focus on the juggernauts. When a juggernaut appears, make that the top priority. It will wreck your walls, and wreck you if it hits you. They are literally and figuratively the biggest threat.
Shoot bloaters as soon as possible. If they burst near the walls, it makes it tough to see if/where zombies are that are banging on the walls, and it can make it impossible to repair a wall until the stink clears.
Coordinate it so that only one person goes inside to protect the technician if a feral climbs the wall. And don't bother healing the technician until at least the third round. You don't want to waste all your bandages on him, but after the 3rd or 4th round you'll need him at max health to make sure he survives. If he doesn't, game over.
End of round tasks: Run out into the field area where the Cleo drops will come down as soon as you can, even before they drop. Grab what I mentioned above, and use the rest of the time to fix walls, switch to another gun if you need to, set explosives, and heal the technician. If you have time/space, drop some things in the storage boxes and run back out to grab more. Anything you don't get will be lost with the next wave. And beware, occasionally ferals will spawn between rounds in the field, although it's pretty rare, maybe a 1 in 10 chance.
With some practice and these tips, you should be able to get this achievement without much trouble.
Lastly, people who downvote legit solutions with a lot of work put into them for no reason are the real heroes here.