never, and let me say it again, NEVER use anything but the bouncier stones upgrade, even if this seems illogic, it's better to have a multiple hitting stone than multiple stones
so, ok, I asume that you never had any trouble in getting to lvl 5 (that lvl 3 can be tricky though)
to beat lvl 5 use your first stone to destroy the first gate, and part of the surrounding (pardon my english) walls
you'll see that you've left two big pieces of castle, the ones that were to the left and to the right of the gate, from now on, I'll call them left and right walls
the second, and possible the third shots will be at full angle (all the way up) and a little less than 2 power, say like 1.75, that showld take care of almos all the left wall, but nothing of the right
now, using full power and a low angle aim to the right wall, the idea is that the stone will hit the right wall, bounce all the way back to the left, and once more to the right, hence getting a lot of hits per stone
do this until you've destroyed both walls, after you finish you must have at least 2+ stones left (I usually have 3 - 4)
now, put the left end of the left line of the crosshair just touching the border of the mountain (that is to say, the left end should be touching the limit between sky and mountain) and use a full power shot
cool, huh? now you only have two little stumps to take care of
lower the crosshair just a little bit and make another full power shot, it should hit the tree but continue to advance until it hits the right tower
rinse and repeat
finally, use a full power, full angle (all the way up again) shot
rinse and repeat
congrats, lvl 5 beaten