To get the Master and Commander achievement, you also need to do the scanning in each galaxy to recover any GAW assets.
If you go to the War Terminal (in the War room with the Crucible hologram) you can check what your military strength and effective military strength is.
In order to unlock the achievement you need to have a minimum of 4000 effective military strength before you begin the assault on cerberus base.
Multiplayer can also raise your readiness rating (for successfully completing games) which will in turn make the 4000 effective rating easier to achieve as well.
