I've found this achievement to be kind of annoying due to accidently falling on bad guys.
Easiest level naturally is 1-1 the first you come across.
It seems to count the treasures you break as well as any enemies, be very careful shooting as the bullets travel and may well kill someone off screen by accident. Bumping into enemies you don't kill and enemies ramming into you count against you as well.
You can shoot , this will not count against you.