How do I lower my hitpoints?

You can reset your effective health points by resetting the amount of candies you've eaten.

In the developer's console, enter the following command:

candies.setNbrEaten(0);

You can actually lower not-yourself's hitpoints:

quest.things[1].hp = 1

...but I found that it doesn't work. It seems like the other yourself is hardcoded to have your number of hitpoints minus 3 (well, 3 was the HP delta for me), so this wasn't useful.

What did work for me, then, was lowering my own hitpoints:

quest.things[0].hp = 5

(I'm not actually positive that such a change will let you win every time; sounds more like a coin throw on paper to me. Should you fail to beat yourself, you can try again immediately with quest.tiredTime = 1.)