This achievement is doable with only two people (and the best equipment). However you need luck as well because in Wave 5 and 6 things could go out of hand quickly.
1, For the gun bring the Cleo Blast rifle (for finishing Wave 3 20+ times). This gun is just plain OP. One shots anything but the Juggernauts without using zoom when you aim in the general direction of the head. Yes the armored enemies as well! If you can line up (for example at the side of the wall) it can kill 3-4 enemies in one shot. Note: we did it with a C Blast rifle and C Shotgun combo as well. The shotgun is very similar to to the rifle but it has a few differences: can't one shot armored zombies and weak against Juggernauts BUT it shoots much faster so tears through everything else much quicker.
2, Melee doesn't matter because you don't have time to melee. Bring the lightest one because it helps with the overall stamina usage.
3, For explosives bring the rocket pods. Many people say the minefield but the rocket pods are much better. One pod can one shot any Juggernaut!
Preparation: The AI guy should have a rifle. If not then restart. Both players will fight outside the wall. One should be on the left the other on the right side. Drop your cleo gun and ammo somewhere close. Drop the first aid pack at the middle between you and the other player, the bombs you find as well but little away to not mix the two. Drop the bandages at the Tech guy, both players can heal the guy with these but you will probably not need it. You never need a bag of snacks or stamina pills, throw them away and never pick them up from drops. At least one player should keep a repair pack at him all the time but you can drop the rest behind the wall. Try to pile up packs as much as you can because there is nothing more annoying than picking one up when you don't have much time and realizing you can only repair one thing with it. One player should use the shotgun from the left box and the other the sniper from the right box in the first three waves. The reason why I suggest the sniper over the assault rifle (Stormbringer) because it shoots bullets (always use it on single) as fast as the rifle but it has much more power meaning it has a small chance to blast off the head of an armored zombie if you can shoot it in the face. The sniper guy has to be mindful with his ammo.
Wave 1: Really easy. Shoot them in the head. As for the Armored zeds either bring them down with a headshot then finish them off or if they are at the wall execute them from behind. Watch out for the feral, The sniper can shoot it through the windows if he runs past you. Later anyone with the Blast Rifle.
Wave 2: Still easy. Bloaters are priority targets now because they gas the area where they die. You don't want to get cornered because of the gas clouds. When the round ends both players should drop everything but their guns from the inventory and run to open two drops (there are 4 total). Exchange ammo if needed.
Wave 3: There is a Juggernaut at the end. Pick up your rocket pod before it comes. The player who is closer should wait for it to get closer then run up to it, drop the pod, run away and activate. One pod kills any Juggernaut no need to check. In the end there are 4 drops again. The sniper should be almost out of ammo and change to the blast rifle. The shotgunner usually still has a lot and can get away with using it until the first Juggernaut (halfway in Wave 4) but can change as well. Ammo isn't such a big deal with these OP guns. This is the first wave where you may see the repair icon on the walls (but realistically the fourth should be). Always repair them to max, no matter the damage. As there are only two of you you don't have time to repair and there are plenty of kits.
Wave 4: More special zombies and two juggernauts. If the Jug is mostly alone then shooting it with both Blast Rifle takes him down extremely quickly (10-12 bullets overall), then one or two shots to the head blows it off. From now on, only ride the kneeling Juggernaut if his head is completely obstructed (for example kneeling in the wall) or there are too many zombies around and no time to shoot two times without getting grabbed. Riding the Jug in the later waves is very risky because most of the time the player ends in bloater clouds or can catch fire from burning zombies. If you don't have time to deal with him one rocket pod takes it out. You probably found grenades you need to detonate after dropping them. These are very useful but you can miss with them a lot. Three and some shooting (or 4 if you don't have the time) should take out a Jug. Use these first because later you won't have time to wait for these weak explosives. After the round you get 5 drops, but these are the worst in the game. Don't pick up the cleo pistols just unload their ammo (Y) and get back to repair everything. Everyone should have at least 120+ cleo ammo when a wave starts.
Wave 5: Probably the toughest wave in the game. Wave 6 is hard as well but this takes the cake because the tons of Armored+ zombies. I say plus because while the Blast Rifle takes off the head and the helmet of an armored one from pretty much any direction and any distance, in this wave this rarely works for some reason. One player probably need to fix a wall (usually the middle) once while the other must line up all the heads on the outside and blast down the dozens of zombies to give some breathing room for the fixer. There were times when I blasted a zombie 6+ times from point blank range with the rifle at his head still didn't pop off. If things look bad then sacrifice a rocket pod to hold the line. Share the 6 drops after the round. You will usually get 4 minefields here. drop two in front of the wall then another two halfway or if you are getting overrun. Note that you get knocked down from your mines if someone detonates it in your face and the detonation detonates bloater clouds as well.
Wave 6: Enemies finally back to normal in this round. Fix everything and share all the explosives and ammo. You can and should use up all the explosives without fear because Wave 7 is extremely easy. There aren't any tricks here just a lot of zombies. Take out the bloaters asap or you won't have any place you can go. The firebombs are useless against the Jugs. After W6 there is a 100% chance you get one cleo smg in one of the pods (usually the left) unloading this nets you 128 ammo on it's own and there will be a lot of ammo packs as well. You don't really need anything anymore but the rifle and the ammo (fix the walls of course).
Wave 7: Is a joke with the Blast Rifle. There are only a minimal amount of zombies here and a lot of Juggernauts. One player kills the zombies, the other one kills the Jugs early on, then both the Jugs when there are no one else. When the double Jugs arrive don't gang up on one together but shoot a different one because the rifle stops them in their tracks so one won't reach the walls while you kill the other.
With this tactics there is only a very small risk of damage to the Tech guy, however when you are out gathering drops there is a chance a feral can get into the base. He could kill the Tech guy extremely quickly. For this reason when you share drops it is much better to someone grab the nearest ones to be able to run back asap to the base both for protection and repairs while the other can take his time collecting the further loot.
A last advice for general target prioritization:
1, Feral who is attacking the tech guy
2, Juggernaut at the wall targeting the wall
3, 3+ zombies bashing one section of the wall (damage ramps up exponentially with more zeds)
4, Bloater anywhere anytime you can shoot it (preferably should never reach the road)
5, Juggernaut in the park / on the road targeting a player
6, Feral on the road / inside the wall but targeting the AI soldier
7, Armored zombies regardless of targeting (it depends how dense is the population, if there aren't many zombies it is better to dodge and shoot the ones standing at the wall, if there is no room then thinning the ones who are coming after the player is better)
8, Normal and Plague zombies who are targeting the player (blinking dot on map)
9, Normal and Plague zombies who are targeting the wall in small numbers (green dot on the map)