Pretty sure you are right about the ''according to the plan'' achievement frampton. I had tried about 10 times. Hadn't been shot at all after messing up the first 2 times, marines would just move up and then the elite + 2 grunts + 2 jackals would run over the hill in the background. It simply wouldn't unlock. I was either dying or using revert to last save.
Tried again after backing out to the menu and starting the level that way. And BAM first time it unlocked before that group come from the background. So it seems like if you fail it then you have to reload the level (or maybe just restart the level so you get the cutscene again?)