My advice is do not use Admob or Facebook because they are both very expensive, I personally use Waypedia.com and here is a coupon code: IRTK5
use it when you register and you will get 10% more funds in your first deposit.
Now it’s been almost 5 months and I always get great results! The trick is to buy High retention installs and not regular retention ones, because with the high retention installs the user must keep the app for at least 3 days and that’s good enough to boost your ranking on google play, I often order more than 1k High retention installs, it’s very cheap it costs $0.16 per worldwide install.
Now you know my ultimate strategy :).
and here is a coupon that I found online: IRTK5
use it when you register.