acorns can be hit several times, which ya most likely knew. I couldnt get 18000 at first either, but I did when I used the reaction command (where the little whirlwinds draw in the rocks) at the very end where all the rocks and acorns come downwards like space invaders. Before that part happens, go close to the top and right of the screen but not all the way and swing rapidly to kill rocks until the whirlwind comes and use it. It gets ya like 5000 points from the acorns, which you can't kill all of them without the wind.