On the 1st level, you must eliminate all enemies but the 2 leftmost bee's (the leftmost being the 2 bees that are all the way to the left where one is above the other).

Let these two bees continue to attack you. Don't shoot them.

Continue to dodge their bullets and after about 2-3 minutes you'll notice that they only shoot 1 bullet each. Keep waiting.

After about 6 minutes, they will stop shooting at you altogether. Let them come down the screen at least 10 times in a row without shooting at you.

Kill them. That's it. If you waited long enough, no enemy will shoot at you the rest of the game.