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I managed to do both. The wall thing I really didn't find that difficult, I actually slid along the invisible starting line at the front and got it done in 30 minutes. Didn't occur to me to slide along the back wall for more time to react. I'd say try it my way if you've been hanging out in the back, who knows what the change in perspective might do for you.
The spear-throwing game is insanely difficult. Took me 3 hours, but some of that was spent taking a break for dinner. I wasn't about to quit because once I got to level 18 I wasn't confident in my ability to ever get that far again. I managed to get through it with a lot of trial and error, picking little landmarks in the scenery to aim at in order to hit each target. For example, for stages 18-20, I used just above the top of the center brick in the arch in the door in the back as a guide, having figured out I could aim there and hit most of the targets at that height by just adjusting left or right accordingly. I recommend trying that if you're like me and your on-the-fly aiming just isn't very good. The real challenge was doing it quick enough to hit the targets before they went back down.
Does anyone know how I can get that William Tell hat, by the way? The information on the website isn't very useful, as usual.