The overall strategy of a marketing campaign is to focus your scarce marketing resources on the task of acquiring customers who will stay engaged in your game. The broad approach, particularly on mobile, is this:
On the day of launch, harness the power of your brand to get featured, and if possible, covered by the relevant press. Rise up the charts, gaining free organic users as your visibility increases. Use your frenemies to send you traffic, amplifying the benefits of this visibility. Keep getting lots of users for free, who, if they are engaged and enjoying your game, will stay and spend money, allowing you to re-invest some of your revenues into the cash marketing that you need to stay visible in the charts, getting more free users. Repeat ad infinitum.