Zakum

In the pre-big bang days the hardest hit was 2800-2900 probably 2800 since someone else said that from zak (and then the 1/1 of course). I'm not sure if it's changed now (but if it has it's gone lower which is only good for you). But I recommend trying to get as much more hp as possible. It should be easy with all the rings and belts and things that we can make now. And the hp scrolls give great results now. So scroll cape, gloves, shield, hat, top, and bottom for hp. Equip some rings (the evolving rings give 150 hp, the zombi rings give 100 and are cheep, the adventure rings give 180 hp if you have one of those). Also the emergency rescue tube is great, and there are other hp weapons as well. And you should be able to get 900 more hp without difficulty.

Dark sight does not protect you from magic attacks, only physical attacks (like if you wanted to stand on zakum). But if you get a magic attack while you are in dark sight then you cannot pot and are in danger of death.

I don't recommend unagi as your hp pot because the damage can be higher than the unagi will heal (so a 1/1 followed by falling fire pillars will kill you). Select a pot that heals more than 2800 hp (cheese was really popular back in the day, but now you can probably make something more precise with alchemy).