I believe the easiest way to beat the final level was simply to exploit the pause button. I simply paused it right away, and then repeatedly unpaused and repaused very quickly. I did this until I just BARELY saw the fireball animation beginning for Grunty. Then I unpaused one final time, hitting A IMMEDIATELY after unpausing, and cleared the fireball without issue. This takes all of the guesswork out of the last level. Let me know if this helps!!