I just got it after 3 tries:
At least 1 objective must survive, and you cannot die between the time you active the devices, and when Dr. Boyd comes on the com and says “Well, there goes my Nobel Prize”. After that point, the defense section of the Op is over, and the cheev should pop immediately.
I suggest going into Matchmaking and doing it. Here’s the load-out I used:
DMR/Magnum/Plasma Grenade (but I swap the Magnum for a SAW)
Autosentry
AA Efficiency
Dexterity
If one person with this load-out stays on the second level of the structure (the balcony-ish area with the UNSC equipment), he can provide support with guns and the sentry, and stand a good chance to survive the defense section without dying.
In other words, it’s not glitched. It just has very vague requirements.