You can actually lower not-yourself's hitpoints:
quest.things.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.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.)