I've played through the game another 10-20 times, and I've learned a few things.
I still haven't killed Satan, but I believe I've figured out what the important factors in the fight are. EDIT: I have now successfully beaten Satan with these strategies, so they definitely work.
The Fight is Buggy
The longer the Satan fight goes on, the more likely it is that the game will bug out and crash. After a crash, you must start the fight over. This seems to be precipitated by Satan's area of effect fire attack killing one of his bats, but I'm not positive. In any case, this makes it extremely difficult to kill him, even if you're well-prepared.
EDIT: The bugs in the final fight have been fixed!
The Wizard is Superior to the Crusader
Or, at the very least, the Wizard seems much better suited to this particular fight than the Crusader. Not only does the Wizard have the mana to deal significant ranged damage with the wand, but a fully upgraded Amyfist spell does a respectable 25ish damage against Satan from almost anywhere on the level. Perhaps more importantly, teleport allows her to escape the bats while continuing to deal damage with ranged spells.
Armor Doesn't Matter Much
Using the Wizard, teleport should let you avoid the bats, unless you're in a really desperate situation. So, for Crusaders, it may pay to use chainmail or plate if you're going to get hit by bats a lot. For a Wizard, only leather armor (for dodge) or robes (for magic resist) make sense.
Thanks to the crashing bug, I have tried the fight with +1 robes and +3 leather armor. The high magic resistance on the Robes dropped the damage from Satan's area of effect attack from 28 to 25. As I had 40 HP, this is pretty insignificant. In both cases he can two-shot me. With a lot of HP and a high +X robe, you might be able to get to the point where you can survive two attacks in a row.
Finally, I think it's worth noting that although leather armor has a much higher evasion rate, anecdotally, I have found that I actually evade more of Satan's area of effect attacks in robes. It may be that magic defense also contributes to evading magic attacks, or I may have just gotten lucky.
Consumable Items are Key
The most important items for this fight are:
- Red, Blue and Yellow Potions for healing and mana regen. Heal meals and gingerbread heads are also useful, but limited by their rarity (and the fact that they don't stack in inventory)
- Nets are great for pinning down bats and giving you a couple more turns to ranged attack Satan.
- Bow and Arrows or throwing weapons, as a last resort (since these are significantly less effective than upgraded skill attacks, or even a wand, which requires no ammo).
Killing Satan Requires a Complete Dungeon Strategy
Ultimately, you have to prepare the entire way down to the bottom level to stand any chance of beating Satan.
- Luck is a factor. It is not possible to reach the bottom on every run through the game. You may not have enough food, and occasionally you may end up in an impossible situation. This is more common with the Crusader, since Gypsies and Ghosts can teleswap you into a mass of enemies you can't escape.
- The locker can be useful, but it doesn't always spawn. I believe the best use for the locker is storing strong weapons and armor so that on subsequent playthroughs you can repeatedly use +1 scrolls on them. Another viable option would be to just stash huge stacks of potions. You have to be really really patient to use this strategy though, as you're going to spend many playthroughs just getting your locker stash built up.
- Abuse the Genie. He will spawn an endless supply of golden beetles, which are very easy to kill as a Wizard with a wand and full mana. Simply stand in the very back of the room, next to the door. The genie can't touch you there, and you can kill dozens of beetles. Although most don't appear to drop items, they will all drop something after the genie is killed. You can easily amass dozens of potions and several hundred gold in this one fight. The only limit is your food supply. Without these huge quantities of potions, you won't stand a chance against Satan.
- Wander Dungeon 4 as long as you can. This is where the highest-exp monsters live, and they will respawn as you circle the dungeon. As long as you have food, keep killing. This is your last chance to level up before facing Satan. It's probably even worth using your heal meals here, if you think you can get another level out of it. As a wizard, you can even keep going after you run out of food, by using upgraded Purify to heal yourself. When you're not fighting, your mana should regenerate fast enough that you can outheal the hunger damage. When you finally decide to face Satan, remember, there is a piece of meat waiting for you at the beginning of his level, which should be more than adequate to cover you through the final fight.
- NOTE: As of November 2010, an additional 4 levels of Hell have been added between the dungeon and the fight with Satan. These levels will probably be a better place to grind, if you gave the opportunity. I may add additional Hell-related strategy here once I have the chance to go through it a few times.