Everyone's solution seems to be a non-legit way using second controllers, which is a totally viable option; but for those who prefer to get it legitimately, here's how I got it, and it was really quite easy.
You MUST have the Crash Course DLC for this method.
I played it alone, on easy mode, and on the Crash Course DLC playing as Louis. It took me a total of 5 attempts; about an hour and a half to two hours. I recommend you play with captions/subtitles on.
The first map NEVER seems to have a tank, and witches are rare; this makes it a very simple map, really. Stay extremely close to your three AI players at all times. Keep moving forward during breaks, preferably quickly, and stopping for special infected and you should very easily make it to the safe room.
The second map is a bit more difficult; there is a possibility of a tank appearing about halfway through. My method for tanks was to RUSH him (after catching him on fire if a molotov/gas tank is available) to be sure that he chased me rather than the AI. Once he came after me, I turned and RAN away; if you turn a corner and the AI stop shooting, turn around and finish him off, but do NOT stop moving: just keep walking backwards and shooting. They die quickly in easy; stay alert and try to always know where the tanks are. Avoid witches unless you're skilled at crowning them.
Once you reach the finale area, search for molotovs and trash then start the generator and go to the balcony that sticks out of the warehouse; the AI will take care of most of the zombies. Help them out a little, but if you hear a smoker, get right up in the AI's business and punch constant. Methods for the tank remain the same, but I recommend staying on the balcony until he comes; the AI may make "ammo runs," despite their unlimited ammo, which you shouldn't worry too much about. Once the tank comes, own him. Start the generator ASAP when it shuts off, that way you can take out the second tank with the lights; they help more than you would think. When the second tank dies, GET THE HELL IN THE VEHICLE!
Hopefully this will help someone; and hopefully it will inspire more people to get this achievement legitimately, because its really not too tough if you have Crash Course. Just remember to move slowly through the maps and unleash your rage on the tanks. They require very little shooting in easy, but don't forget that one punch is enough to ruin the achievement. Get him chasing you then RUN like a noob. If you have trouble, there's no shame in the two controller method! Do what you gotta do!