| "I once thought I could save the world... now look at it"
— Spoilers for Deus Ex: Mankind Divided follow!
In order to save Singh, the gold mask-wearing mercenaries have to be taken out quickly. This is possible in both lethal or non-lethal.
For lethal, simply go in and take out the mercenaries as quickly as possible.
For non-lethal, a quick approach is for player to make their way to the helicopter in the middle of the atrium as fast as possible, while avoiding the mercenaries (using cloak and silent running really helps here). Disable the helicopter by removing its battery, ending the mission and saving Singh.
If you're having trouble making it quickly to the chopper, you can shoot Singh with the tranquilizer rifle or do a non-lethal takedown on him. As soon as he goes down the enemies will stop focusing on him.