Just did it last night and it won’t unlock. My question is, do I have to save the first set of marines and wait for Foehammer to drop that warthog before proceeding? I just walked passed all that fighting and went straight into the underground area.
Weird. Sorry to bump this, but I was just playing Halo CE and the achievement unlocked just like 15 minutes ago. So I went into my achievements and it shows that I apparently unlocked it 15 hours ago. What a strange bug. Guess the issue is resolved.