In the level "Halo" in Halo 3, right near the beginning as you come down the hill and see the large structures across the ravine, but before you drop down. There's a tunnel off to the right that leads to one of the terminals at the end to the left. Opposite this terminal, there is a large gap with a span on the other side that you cannot jump to, not even with a grenade jump.
Have a co-op partner stand at the ledge and stand on top of his head. Then have your partner jump into the gap toward the span. At the peak of his jump midway across, jump off of his head and you can reach the span.
From here, there is an alcove to the left with a half-naked guy floating in his boxers. This is one of the Bungie devs, and I think it's one that is still with them on the Destiny project based on the achievement name. You need to do a 'late jump' to reach the alcove because a normal jump will cause you to bump into the overhang and fall to your death. Make sure your partner has respawned on the span with you before attempting the jump in case you fail.
Once you make it across head over to him and look at him for a few seconds up close to unlock the achievement.