I'm pretty sure the galactic readiness is through multiplayer.
You can also get it from the mobile game (which I have never played, so I can't tell you if its any better than doing MP). If you're on PC you can also just edit your coalesced.ini to make a war asset worth a lot more and get your EMS up that way.
Bottomline though, I believe you only need around 4000 EMS to get the best ending, and I was able to get way above that without doing any MP this time around. I have heard that it's very difficult to do if you don't have custom ME1/ME2 import choices as well as a strong ME3 playthru.
Everybody good, plenty of slaves for my robot colony?