Enter the NOD side tunnel, take the first exit to the right. Have at least 150-200 armour and your ion cannon beacon ready.
Now run at the backside of the monolith up to the entrance of the hand of NOD. Never stop or care for anything. Approach to the pedestal, deploy the beacon, change to your gun and retreat to the back of the building, shooting any incoming enemies.
The ion ray will not even kill you at this position, as you wait for the win screen.

That's the way to get behind the obelisk!