Master and Commander Achievement in Mass Effect 3
Master and Commander50
Deliver most of the Galaxy at War assets to the final conflict.
How to unlock Master and Commander
For this you will need to have over 4000 "Effective Military Strength" before beginning the mission "Priority: Cerberus Headquarters." You will gain assets through playing the main story, doing side-quests and scanning systems to retrieve resources (see "Lost and Found").
It is possible to do this at 50% readiness (without playing online), but you have to be extremely thorough and it will require 8000+ Total Military Strength. On top of that, some assets won't be available depending on your choices from ME1/ME2 so you may have a game that you made some bad choices and can't get some larger bonuses which would require you to play the entire trilogy over. You need to play the MP anyway to get "Defender" so you really should invest a few hours into the MP to boost your Readiness.
To make the achievement much easier, simply play a few games online to increase your Readiness Rating. If you do not play online for a while, your readiness percentage will begin to decline. I would suggest doing the "Defender" achievement right before this one, that way you've got a full 100% readiness and will be plenty over the required amount.
To clarify, the bottom number in the image below (with the accompanying green bar) is the one you need to have above 4000. The Total Military Strength will get reduced by your Readiness Rating which will equal your Effective Military Strength (TMSxRR=EMS). If you complete the "Defender" achievement you will get 100% for the Readiness Rating which will give you full use of your Total Military Strength. It will equal your Effective Military Strength, which (assuming you have 4000+) will net you the achievement.
And again, you can not get the full 1000 Gamerscore without buying the Online Pass and playing the MP portion of the game ("Defender" achievement) so there is no need to kill yourself trying to find every possible War Asset available. Just play a few games online to get your Readiness up and get more use of your assets.
Note: If you have downloaded the free Extended Cut DLC then the amount of Effective Military Strength you need will be cut to 3100, making this achievement much easier to get playing just the single player campaign.