On the mission 2-1, Train Station, there will be several crossbow guys on top of the crates. Sheva will get distracted by the dogs and manjini on the ground so if you climb up to the crates and jump over to the crossbow guys, she should not notice you.
Now this is one of the first levels the crossbow guys appear so they will take awhile to shoot you and will often times just stare at you. So what you do is walk up to the crossbow so that when you swing your knife it hits the tip of the crossbow but not the guy. So in other words you are close, but not close enough to hit the zombie. Now just continue swinging your knife and eventually he should shoot and you will deflect the arrow. If it doesn't work the first time, try again it will work eventually. Good luck.