The key to this achievement popping appears to be more of a glitch in-game, not of the achievement, but of your health. When at the final battle, if you immediately go into the foxhole, even though you don't lose a heart, it does count as a "hit". Walk away from the foxhole, and ensure you aren't near that first cannon shot. After several attempts of jumping right in, I did not get the achievement. The first time I did not just jump right in, it popped without any issue at the end of the battle.