Step 1) Start Infinity at Rally Point Bravo.

Step 2) Try to find some good weapons, headshot weapons, pick up an autosentry. Give the marines some good guns.

Step 3) Protect the marines at all costs! Run a head of them, try to keep the fire on you. Make sure you kill every enemy. If you lost a marine, just restart at the checkpoint.

After Advance all the way through, without one preventable marine death. The achievement will pop up.