SMT: Vengeance Satan Hard Mode Quick Clear (New Game, No Sutras/Incenses) 8 Turns

Omagatoki: Charge is good.

IMPORTANT: This strategy can also be done with a base level Norn provided she has both endure and during soul. If you do this you may need force sutras or magic essence on Baal.

The Satan fight has heavy scripting and it generally goes like this if he's allowed to use all his turns and none of his summons die.

Turn 0: summon allies then megido ark.
Turns 1 and 2: almighty move (either megido ark or megidolaon), his allies move, then he does another almighty move.
Turn 3: Magatsuhi plunder, remaining allies move, charging the magatsuhi skill of whoever he ate (this takes all his press turns no matter how many he has).
Turn 4: Using whatever Magatsuhi move he charged, any remaining allies move, then call allies.
Turn 5 onwards: repeat from turn 1.

If you interrupt his press turns during turns 1 and 2 then carries on with the script as normal. However, if you interrupt him in turn 3 or 4 then he will use whatever move he would have used on his previous turn, had he been able to use it. For instance, if he uses accursed poison as a Magatsuhi move but it costs him more than one turn (say you're using Demi-fiend who blocks all ailments) then he will use call allies on the first action of his next turn.

While it's not necessary for this video I should also mention that he will use call allies on turn 1 or 2 if you killed one, I don't know how that interacts with the rest of his "script".

With this knowledge and the chaotic will skill from Demi-Fiend to half his press turns, we're able to significantly draw out how long Satan spends turns doing idle (Magatsuhi plunder, using a useless Magatsuhi move like shield of God or Dekajaon, or using call allies). This gives us enough time to charge up the Magatsuhi bar.

We also rely on an AI quirk that has to do with the 100% dodge skill Luminescent Mirage. Presumably the devs didn't want the MC using taunt + dodge to avoid megido ark so if you use the dodge skill on turn 1 or 2 then he will always megidolaon instead. If we use chaotic will to half his press turns when he uses megidolaon then we only have to deal with one attack on turns 1 and 2.

The party order is essential here as are the exact demons used (Baal can be swapped for figment slash Odin but he's much less reliable). Yoshitsune returns from the Demi-Fiend fight to deliberately miss and proc his innate skill, Norn's innate skill is used to grant an extra half press turn on the charge turns, Beelzebub's innate skill is used to get twice as much value from debilitate, Baal is a good high level Deity with force affinity and
only Demi-Fiend has chaotic will and he does good damage as well.

As for the fight itself, megido ark will kill anyone on turn 0 so endure/enduring soul is a must for all your demons. I would have put it on MC as well but he needs all his skill slots for other things so I had to make do with a whittled goat.

It is essential that we kill Hariti before turn 3. This is because it seems random which demon he eats (if you know what causes him to eat which demon please tell me) and him using waters of youth would obviously be very bad. It would also be bad to let any of his summons ever get an action because they like to use (de)buff skills or dekaja/dekunda.

MC's skills:
Figment Slash
Evergreen Dance
Luminescent Mirage

Critical Zealot
Both Phys Pleromas
Dragon Eye
Safeguard

I'm not sure whether beast eye would have been better than safeguard, I never tested it but I suspect beast eye may be better.
I should also mention that I did not max out strength for this strategy, the MC has 170 strength and 119 agility to help with the hit rates. Again, I'm sure what the exact balance is between strength and agility but what I did works. If you had so much strength that Satan would be brought below half during the first use charge turn then he would summon 3 allies next time, which would completely ruin the strategy. You would need a lot of strength for that to happen so I doubt it would ever be an issue.

Other important skills:
Great Life Spring on Baal - this is so he can survive 2 megidolaons. Endure would also work instead.
Both force pleromas on Baal to let him kill Hariti.
Boon Boost and Boon Boost EX on Beelzebub and Norn.
Magatsuhi Wellspring Miracle- MC doesn't have enough MP otherwise (this wouldn't matter if I had +5 support affinity but I didn't have enough glory).
Every relevant Magatsuhi generation skill because the bar is really tight.
The Prayer of Magic and Vitality miracles were also used.

Of the two SMT V superbosses I've fought I'd say Satan is easier than Demi-Fiend if you were to fight them both without any kind of "cheese", mostly because you can't use chaotic will against Demi-Fiend and Satan spends a lot more turns idle. If you were to fight Satan without bursting down his health with omnipotent succession then I'd recommend Sakuya or a waters of youth demon.

If I hesitate for a few seconds then it's because I forgot what to do.