Press "P" To Fight The Final Boss

The Demon Lord must be slain, but that's a lot of work.  You must first find monsters in the grass and then defeat them to level up, but that sounds boring.  Thankfully, the Hero has the special ability to do all of this while asleep.  Simply sit back and relax as you fight in your sleep and when you feel you're ready, press "P" (while on the map) to go fight the final boss.