How Do I Get Past Level1?

The best strategy is to circle around the Nibblers which will form them into a big clump, do this  over and over again until the progress bar at the top left reaches 100%

How Do I Beat the Boss?

There is a trash can in the corner of the screen, you can drag it around with your mouse. Take the Trash can over the bosses head and release it. If lined up correctly it should fall onto them, do this 5 times in order to kill the boss.