(: took me like 20m
For 8: Use 1 move to slide into position, use another move to jump onto the ledge, use another to jump onto the top of the small box, another to jump on top of the larger box, and finally one more to fall into the green square. For that final move, it's basically just luck.
For 11: Use 1 move to jump out of the box and into the green squares. If you're lucky, both of your cubes will land directly onto the squares without any extra work.
For 12: Use 1 move to jump into a box (on either side) to make your cube larger. Use another to "duplicate" your cube into 2, and pray to God that one of your cubes lands in the green squares automatically. Use your 1 remaining move to get your final cube to the green square.
I got so close on one of my first few attempts with your advice on #12, then I botched the third move. Now I can't seem to get lucky again.
Edit: finally got #12 after botching shot #3 a second time, then getting it after the third time getting lucky. My brain needs a break now, or at least the part of it that's optimized for physics prediction...