Some tips and interesting facts about this achievement:
-Unlike Left 4 Dead, you don't need to be alive when survivors escape. If you are dead and your team escape, it counts to you too.
-You can join a game that it's already started. Is not necessary to start the game from the lobby.
-An obvious tip: try to play with people you can communicate with. If you play with random people, even though it's possible to pass the game, you will probably fail. I did in in a party with my brother playing with 2 cool french guys that were in their own party. But we were sending each other some messages some times. If you play in pairs, like I did,but cooperating the 4 of you, the game can be pretty easy.
-When people are shooting in a closed place, if you are in the front line, always coach down to let the people behind you to shoot and to avoid friendly fire (which can be deadly in insane).
-About classes. 2 medics and 2 fixers at level 30 should me more than enough. The perks to use are very subjetive, each one could prefer different perks, but if you want to know the ones we used, let me know. Obviously, use the class wisely. First aid kits should be always for medics and things like that. This is one of the biggest problems playing with random people.
-Stealth / Speed run. Use silencers to pass through some level parts, it makes some parts way easier. Other option is to use fixers grenade to became undetectable for a while and be able to pass parts in just seconds.
- Defending objetives. One of the worst thing playing online with random people is that they tend to forget about objetives and they only focus to stay alive. There is no sense on staying alive if zombies will take the objetive. In extreme situations, give your own life (or deffend it with masked effect) to deffend the objetive. If that makes you to pass the chapter, it's worth it (remember that you don't need to be alive). But if the 4 survivors play good, this is not neccesary.
-About weapons, you can use the one you prefer. Bit be sure you got the highest version of it. And try to avoid melee combat as much as you can.
-Don't be impulsive or impatient. Stay calm and move slowly.
-Learn where special zombies respawn. After some games played, you can learn it and it can be very useful.
The more difficult chapters for me were the second one of Moscow, the first and second one of Jerusalem and the first one of Tokyo. But after some attempts you start doing it better until you finally pass the chapter.