*SPOILER*
Here is some help; the beggining part is the hardest one, if you complete it you'll finish the level sooner than you think:
You will need 13 boxes to complete the level; 12 to build bridges to get the coins (6 each) and an extra one to cover the red button near the exit.
The boxes marked in RED are the ones you need to destroy.
After that, you collect the coin marked in green to make the Z-bot press the timer button.
*Remember: The 2 UFO's walk at the same speed starting from the same place, so you can always know where the other one is if you watch one of them (in case the high walls cover your sight).
The rest is for yoiu to find out, I don't want to make things too easy.
SPOILER ENDS HERE*