Make the green player go left once. Get the blue player to go up three times, then right once. Move left twice to drop the rock.
(Blue Player) Move one space up, then one space right to drop the bomb. Go down twice then right once to get a crystal, then up once and left three times to get another. Go up twice then left once.
Now for a bit of "sweeping". Keep going left until you reach the wall, go up once then keep going right until you reach the wall, then go up once then repeat. Keep doing this until you're under a rock on the left wall. Move right, then left, then up then resume sweeping until you're under another rock. Move left, then right.
Continue sweeping. Stop when you're under a huge pile of rocks. Go right once, then wait for all of the rocks to fall. (There's a second load of rocks that fall shortly after the first, so be careful.)
Go right until you collect a crystal. Some rocks and a bomb will fall. Start sweeping again until you're under a rock again. Move right, then head up then left, and you've collected the last crystal. Level complete!