30 Jul 2013 03 Aug 2013
From your main menu select Factions, this will be the multiplayer portion of this installment.
Now choose Fireflies. (don't worry this choice isn't permanent, you can change once you die or complete 12 weeks 7 days.)
Once you have made the choice of faction '12 weeks' begins. You will need to survive for 12 weeks, roughly 80+ matches. Survive does not mean win 80 matches, but rather to have a population after the 12th week and 7th day. Your population is directly related to how well you did in a match. Kills, revives, crafting, all reward different amounts of supplies, and these supplies correlate with how healthy your population is. You must have at least 1 member alive by the end of the 12 week 7th day for this trophy to pop.
Throughout the weeks of gameplay 'Missions' will pop up, they will either award population or decrease it. These missions will require you to do certain things during 3 days (3 matches). They have three tiers and you can choose from a variety of requirements ie. 2, 4, 6 executions by the end of day 1,2, and 3, respectively. Now here is the tricky part, on week 6, 8, 10,11 and 12 there is a 100% population wipe mission. You MUST pass the first tier requirement of these missions or you will have to start over your 12 week cycle.
I completed 24 weeks without ever wiping my faction, and acquired both achievements for firefly and hunter.
The way I did it was to save the easiest mission requirements for those specific 5 weeks. If a population boost "friendly" mission comes up, use a hard requirement, or ignore it entirely. You will need the easy requirements for the 5 week MUST pass.
Now this is opinion in which are easy and hard compared to your skill level and gameplay style. So choose whichever you like. But a few are unanimously "easy," I would recommend to save these two:
Each of these can be done twice, so that covers 4 of the 5 weeks that require a MUST pass. The 5th week to cover is up to you, some easy-ish ones are:
-Specific weapon downs
In summary, remember in Week 6, 8, 10, 11, and 12 there are 100% population wipe missions, these are the only missions you should be concerned about, every other one will not cause a population wipe.
Have fun, this is a beautiful game!
I have heard that if you are on a MUST pass 100% population wipe, on day 3 and you are about to lose it (not passing the first tier) you can dashboard and it will start your 3rd day over. (I did not have to do this so I cannot confirm or deny that it works.)
 Please check the comments for a complete list of mission minimum requirements. Thank you lostmantella9