The tunnel is the only one I'd consider very difficult or difficult at all, so I just don't bother with it, but it's not "impossible" (though I'm not certain I've ever seen a clean shot through it either). I can make all of the other shots the vast majority of the time and they are all drastically easier to make. Generally it's easier to make every shot when the crossbow is on its way back to the left (at least for me on my machine).

I've never seem it hit clean...tower is no easy shot either..for me anyway

Tower is easier off the walker bank shot. Believe it or not you can practice that and get very good at banking off walker from left flipper and crossbow. Tunnel is possible also just very hard much like it's very hard off flippers

The tunnel shot is the hardest. What makes it so difficult is the speed of the crossbow motor. Since the crossbow is moving left-right so quickly it adds a slight "curve" to the ball trajectory, and the tunnel shot works best with a very straight trajectory.

Here are the best times to shoot the shots - while the crossbow is moving to the Right or Left (left is the return)...
- tunnel - right
- right ramp - right
- riot - left
- BG ramp - left
- barn - left

Should have thought of that since i get a lot of that from straight up well walker hits..

I can make the tunnel shot about half the time. If you time it perfectly and your level on the machine is not too step, it should be able to make the ramp most of the time.