If you want an easy way to get this...play through the tutorial stage once, and if you beat the knight, great, you don't need this guide.
If you fail like I did, then right after you lose, or win for that matter, the game lets you buy some things, and you have enough coins to buy them.
Buy the green potions and the shield that you can buy.
Now simply play through the tutorial level again, and try not to get hit while making it to the boss (but it honestly doesn't matter much).
Press Y if you are low on health, and continue hold down X to attack him. You get 5 health potions, so you should be able to beat the boss with them, just avoid his attacks as much as possible.
Once you get the achievement, start over if you want so you get your potions back.