This can be difficult to pull off solo, as I got my first five done alone. There are 5 in every regular playthrough. One on the Crashed Plane Site and 4 in the Mian Streets of Marshal.
You will need 15 Plague Busters but six are given to you by Logan when assaulting the first one on the plane. You may leave the destroyed wall site with three to none. Look for bloaters for samples and craft Busters back at base.
What I did was help random players on LFG on Xbox and used my existing community (You can bring your existing community from Vanilla/Base Game) Bring lots of ammo, health, stamina, lure, explosives, gas, and etc.
Automatic Guns, .50 Cal, Auto Shotguns and some sorts of explosives are very helpful, lures are extremely useful to get the regular plague zeds lured away from you.
First wall is easy on its own but I'd recommend 100 rounds of bullets and some lures.
1. Bloaters and Screamers lurk around the primary Gauntlet wall but if you are unlucky there could be some ferals or juggs that can join the battle.
2. As you are getting into the gauntlet Juggs and maybe a feral will spawn along with a screamer will push towards you.
3. More freaks will spawn as you are assaulting the third wall
4. This last wall can be very difficult as there can be about 20 zeds, 2 juggs, 2/3 ferals, a screamer or two, and bloaters. Destroying the final wall will kill all zeds.
Note: Zeds can spawn after a third of the wall's health is lowered so be aware when assaulting alone or in a group. Hence why I recommend you do it with others as it makes things easier minus the terrible server the game has. Expect lots of resources used when aiming for this achievements .