Is that without playing multiplayer, though?
I had to look this up because of the complexity of the details, but here's how Galactic Readiness, Effective Miltary Strength, & Total Military Strength work.
Readiness Rating & Effective Military Strength
- Total Military Strength is just the total number of War Assets you have acquired. These are collected during your choices in the game, by completing side-quests, how you solve problems during the single-player campaign, and through exploration of the Galaxy Map.
- The Readiness Rating can be increased through online co-op play. It starts at a base 50% rating that can be increased. The higher the rating, the fewer War Assets you’ll need for a perfect EMS score.
- For example, if your Readiness Rating is at 100%, you’ll need half as many War Assets. The Readiness Rating works with a simple algorithm. A 50% Readiness Rating multiplies your Total Military Strength score by .5%.
- If you play the co-op multiplayer and increase your Readiness Rating to 75%, your Total Military Strength score will be multiplied by .75%. Your Readiness Rating will increase through online co-op play, but will steadily decrease as time passes, giving you a very narrow window to play with a 100% Readiness Rating.
- There are enough extras, collectibles, and side-missions in the single-player campaign to earn the maximum EMS rating without having to play online.
So yes it's possible to get the best ending without multiplayer (multiplayer just makes it easier). Though I believe you might have a slightly harder time getting the best ending being that you're not using your ME1 & 2 save data.