Ring of Pain

As the description says, reach the boss with less than a fifth of your total health.
I recommend playing a run on normal mode, and prioritising getting a large health pool. With higher max health, you will have a bigger window of health to aim for when trying to lower your health at the end. Then aim for whatever other method of getting strong you feel comfortable with, so that once you are at low health, you can safely finish the last level before the boss without dying or needing potions.

I went for the path of dark ending, as I don't know what counts as the boss in the light ending. If going for the dark ending and the boss fight against , then when you reach the ? level after level 15, you need to carefully look for ways to get your health down and still finish the level.
I used had over 40 health, and luckily there were two of the large explosive creatures that chase you and blow up for 20 damage. I used them to get me down to low health, and take out some enemies at the same time. There were a few enemies left at the end, but I lit the candles and got to the boss safely.
You could also collect items that lower your curse resistance, and either hope for curse potions in the final level before the boss, or keep hold of the scroll that makes two curse potions appear. Curse potions may be a good choice as they always leave you on 1hp minimum
Make sure not to get so powerful that you can't easily get yourself to low health. I'd avoid items that heal on kill or attack. and since you may need to pass creatures to get them to attack you, maybe avoid reflect items.