Leaders are the people who are not wearing helmets, and have names when you hold the crosshair over them. First, kill all the enemies in the area. Try using explosive crates to your advantage, and grenades. Hide behind non-explosive crates by crouching. When you need better guns, use your invincibility to run out and grab some of the dead guards' weapons.

When the only guard left is the leader, try to find a scoped weapon among the dead guards, using your invincibility 'eye ability' to pick them up.

Then crouch, giving the leader a smaller target, and shoot at the head of the leader. If the scope becomes too wide, hide behind the crate you were using as a shield before, and let the crosshairs reduce in size again.

Try to let your health fill up again, and use your Invincibility to your advantage.