If you want this as quickly as possible, the technique I used was to get up to about 20 survivors, then quit 2 matches then play 1, then quit another 2 games, this keeps the survivors up.
When you get a mission that is not 100% risk, I ignored it, put it on a mission that I wouldn't complete at all, then when survivors died get back up to 20 then start the cycle again. This saves the easy missions for the 100% risk events.
This does ruin gameplay for others slightly but at the time of writing, people normally replace quitters fairly quickly. I played on Supply Raid due to matched tending to be a bit quicker and more enjoyable (to me) but I'm sure you can do this with any game mode as long as you are getting a good amount of supplies per game.
Of course, I would prefer comments over negative votes, as it is way more constructive.