This has been driving me mad. I think the agent I am trying to protect has killed me more time than anyone. I have cleared all the bullets out of the way if Sanderson and spared the guy being killed by the grenade. All that's left is to deal with the guy who comes out of the lift at the end. I've hit him a bunch of times with bullets and he always survives. Even though he bleeds he just runs off and I can never kill him. Eventually I screw up and he either kills Sanderson or Sanderson kills me.
This one annoyed me too. I used the grenade in the end. Once the final guy is left and pulls out the grenade, take off his armor and then take the grenade. The main agent will kill this guy so u don't need to sorry. Teleport over to the other agent and drop the grenade. By the time it detonates the man will have come out of the lift and the blast should hit and kill him. Doesn't matter if you kill the other agent.