In Level 8: Intruder Excluder, right at the beginning of the stage, grab the stick thing and reach an area with some yellow balls, by killing them, you get 750 points each, so keep killing them with the stick until you get 999999 points, just rewind if you take damage. After that, go to a new Battletoads save and keep playing the game until it pops, for me, it popped on Level 2: Wookie Hole.
Also, by killing the balls in Level 8, you'll also get the 500 enemies achievement due to them counting as enemies