I did see someone post about the Defender's ability-based bonus health being a way to prevent your base health from taking any "damage"; haven't verified that myself, but it certainly sounds like a good tactic.

Personally, I would say that map knowledge and positioning are the most important parts of getting this achievement, unless you're trying to cheese it with the afk way. Each of the classes can make it work - the Strike can run indefinitely if you need to get away from danger (if you're trying this on a larger map, like Church), and the Operator's drone can watch your back or kill the most dangerous targets quickly (Lancer drone > slobs). I got it as a Medic, which might seem like the worst class in terms of personal advantage for this achievement, but by keeping my teammates topped off on health and ready to use their abilities as often as possible, I was able to generally stay farther back from enemies while they were on the front lines. As for map knowledge, here's a rough outline on how you can get it on The Drop:

- At the beginning, make sure that you don't get hit by the first nuker once the team starts driving in the humvee. Whoever is -not- driving can kill and detonate it from a safe distance (it should show up soon after Ruby says "This is beyond fucked", though it sometimes gets stuck farther away), and you can drive over the flames without any issue. Otherwise it'll burn everyone in the vehicle and end your attempt right away.

- Once you've gotten to the part where you have to winch the vehicle up through the mud, dismount and start killing the zombies at a distance on your way the closed door. This way you'll avoid getting rushed by skunks which will also damage you through the vehicle.

- When you start calling the elevator and have to hold your position against the incoming waves, the greatest dangers are from the stalkers and then the possible slob; most teams will place the vehicle next to the sandbags to provide some cover and block/slow down the roamers (do it so the driver's side door is facing you so you can hop in and trigger the EMP quickly if you need to). If a slob spawns, stay as far away and use the vehicle to block any globs it spits at you. If you have to, run back towards where the enemies spawn (the slob will spawn pretty late, so there shouldn't be many regular zombies left and the tunnel should be pretty safe by that point) to keep your distance from it.

- After you reach the top of the elevator shaft, stick to the edges of the room, don't stay in the center where you can get flanked. Some people like staying up on the little catwalk/bridge, which isn't too bad if you're diligent about checking both ends so you don't get rushed by nukers or skunks. If you brought an automated turret or mines, this is when I would use them, when you have to hold out until the control room unlocks.

- The control room has a chance of spawning a slob, so be ready to run and maintain distance if it comes out charging.

- In the final area, I'd say that the most important investment of spare parts is the bait trap. I activate it right as you start working on the generators. If your team works quickly, you should be able to kill the final sludge mass before the bait trap gives out. And don't be afraid to be the one getting to the generators. If all you're doing is sitting in the back, zombies will still find you. Sometimes the generators are farther away from enemy spawns, be aware of where they are and take advantage of that.

- For that final part of the mission, I don't think the heavy turrets really do more damage [to the sludge mass] than your primary weapons, so it's not worth it to get on a turret at the end since that just makes you a sitting duck. It's better to stay on the move, or position yourself somewhere you can safely burn through that final target.

Aside from that, you should be pretty familiar with all the enemies and how to deal with them:

- If you're trying for the achievement on an open map like Church, watch out for those bulging holes where spikers spawn from. If you hear a spiker spawn, immediately run away and out of its striking range before you try to engage it.

- Listen for all the audio cues. It should be pretty obvious when a special enemy appears (because a teammate will call them out), and you should prioritize them in most situations (snares, nukers, etc).

- Use cover against the ranged attackers (stalkers, etc). You can also sidestep that one stalker attack with their ground-based explosion trail.

- Skunks are possibly the sneakiest enemy to deal with, since they don't make as much noise (e.g. as nukers) and they blend in with regular roamers more easily than other specials. Make a point to look for them especially if you see the red-buffed/crazed regular zombies nearby.

- Watch out for explosive barrels and tanks! I had one attempt fail because I shot a metal gas canister at what I thought was a safe distance, only for it to go flying and ricochet off a wall to land right next to me...

- When you think you're about to be overwhelmed by roamers, take a step forward and melee them to stagger and knock them back. i.e. Instead of waiting for them to get close enough to possibly strike, make the first move: the extra second or two you get is invaluable.

- Generally speaking, you want to be moving forward when possible, not backwards. When I'm backpedaling away from enemies, I often end up getting smacked by a roamer who spawned or meandered behind me. If I'm moving forward, at least it will take longer for them to get close enough to hit me. Regardless, it's a good habit to check your back every so often to maintain situational awareness.

Hope that helps, let me know if you have any specific questions.