to easily beat the last boss, kill the purple knight, then die in the fires that come up and you will keep getting 400xp. keep doing that until you have the best sword, around lvl 7 armor, and around lvl 8 magic, and you can easily beat the last boss. The more you upgrade the sword, the easier it is to kill the purple knight. Make sure you can take 1 hit from the knight because the fire kills you slowly...
This game is pretty good, 5/5, but i think when a round is over, it should show how many gummys you killed... Also a way to see the distance from the next gate would be nice...