An easy boss level is "Botanic Panic" which is just under and to the left of the shop. You can even choose the simple difficulty level.
First, you must defeat a giant potato. Just keep shooting and jump over what he shoots out of his mouth. He will shoot in bursts of 2 or 3 and then pause.
Next, you must defeat a giant carrot. Avoid the smaller carrots coming from the sky and keep shooting. Once his eyes change you know he is about to shoot 2 beams. Run from them and proceed to shoot him some more. A good tip is to use
which makes it easier to aim.
Once you defeat the giant carrot the level is over and your achievement will pop.