In the Lincoln tunnel mission, at the point where you have to protect the jtf dude while he disarms the bomb, you have no business being in a challenge mode mission.
Also, kill the first wave and leave one guy alive. Wait for the timer to run out and finish him. Easy life equals easy high end.
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I love it when you rush up to stake out the wall with Smart Cover, drop your pulse and then discover the other team mates are hiding back at the ammo back. [spoiler]an agent has left your team[/spoiler]