Back 4 Blood

As of the update to go live this morning, all characters will be avail to play from the start
The earliest you can earn this is after completion of Act 1: The Devil's Return, that's when you unlock the remaining 4 base game characters; Doc, Hoffman, Jim, and Karlee.

**At this time solo play does not count towards achievements and thus you must complete maps in co-op campaign or quick play online**

Post this point, you can either play and complete a map with each character through regular play or the fastest method is to speed run the very first level.

If you complete the level whilst not killing any Ridden you can also nab

Each character run takes about 3 mins barring any complications.

easy mode for this one is just put on stamina and run speed cards, and run from the start of the first map to the first safe house once there and its loaded up the menus go back to base and start again gong though all the characters, it will take around 2-4 mins a run dependant on special infected spawns dont worry about killing just run past things if you can

In this video guide I will show you how to complete a mission with each cleaner. This can easily be on on Act 1, level called Resurgence on Recruit difficulty. Just run through the level and don't kill anything or at least try not to. Not killing enemies will save time in the long run. Just remember to play with a friend to unlock the achievement and change characters when the level is done. Once the last character you need is done the achievement will pop in the saferoom.