Here is a quick video on how to get all 18 holes - keep scrolling for screenshots on how to get each one.
In each of the screenshots, I've captured the exact frame JUST BEFORE shooting every time. I've drawn in the angle on the holes where it needed to be more precise, but if there aren't any notes or alterations, just copying the angle shown will work. Have fun beating Brian!

Just hit straight ahead.

PRE-PATCH you can just hit straight ahead; since the update it's a little trickier. Hit slightly to the right.

Hit straight ahead.

Try to skim the very left edge of the front wall. Get it as close as you can and the ball should bounce around a little before going in.

Hit the top of the wall on the right closest to you. The ball should go straight around the corner and into the hole.

Before the patch, the hole used to be on the other side of the cross wall - but the same angle applies. Try to aim for right between the cross and the outside wall to get the right angle.

Since the update this one is a lot easier. Just aim for the wall on the left as shown and you should get it.

I found this to be one of the tricker ones, but my method is just to hit quite far on the left, like with hole 8.

REMEMBER TO SAVE AFTER THIS ONE!

#Just try to hit a teeny bit to the right and it should go in.

Super easy; just like with hole 10 the ball will follow the pattern and go in pretty easily.

Again, like with 10 and 12, just let the ball follow the wall pattern.

Pretty straightforward but annoyingly easy to mess up. I'd recommend not shooting right away and taking your time to make sure the angle is totally right.

Try to hit DEAD straight. Hitting a tiny bit off centre is usually also fine but can make the ball miss by a super small amount.

I hated this one, but after a couple of tries it should be easy enough to hit the ball in a straight line to the hole.

I don't have a super guaranteed method for 17 - but as shown I generally try to hit between as many of the pegs as I can and most of the time it should roll in no problem.

The important things to remember about this one is:
DON'T WORRY ABOUT THE MOUTH. It doesn't matter what position it's in at all.
Only worry about where the spinning bones are. Make sure they're out of the way.
Shoot very slightly OFF centre, and the ball should roll up the skull and light up it's eyes. It takes a couple of tries but one you know to basically just watch out for the bones to move and which bit to actually aim for it's a lot easier. Good luck!