After some gaming, I've found the absolute boss. This is the end all be all of SRPG and RPG bosses in video game history. The alpha and the omega. A weapon of unimaginably destructive power. This guy makes Ruby and Emerald Weapon weep like babies; he has thousands of Uber Prinny Baals serve him; he is ultra-freaking hard personified. It doesn't get much harder than this (if that's possible).
While not an SRPG in a strict sense (and probably the least SRPG-ish in the Megami Tensei franchise), this one boss is too freaking hard and complicated to even consider it a standard RPG [read: those without strategy and preperation WILL die, 100% of the time, 100% sure]. It's a familier face from Shin Megami III (Nocturne) making a cameo in Digital Devil Saga: Episode 1.
THIS DUDE (and one of the most awesome characters I've seen)
The Hitoshura (Japanese) or Demi-fiend (English).
(http://i58.photobucket.com/albums/g246/Cashino/Hitoshura-00.jpg)
Why? Here's the load down.
1) He has one attack that does about 10,000 damage (and remains stable there), which he uses regularly under scripted circumstances.
...
Why's that scary? Here's a list:
a) It hits the whole party.
b) It's considered Almighty (IE: it can't be blocked, resisted, reflected, or absorbed; it can only be avoided when ONE skill activates; otherwise, you get hit practically without fail, if ever at all)
c) ~10,000 points of damage is the amount of damage when Defense has been maxed out.
d) The players' characters have their HP capped out at 999. You draw conclusions.
e) The only way you can avoid it is using a skill that makes you avoid all attacks when you're sleeping. You can only be effected by sleep by the enemy; you can't cast sleep on yourself. This means you need to rely on luck to avoid this attack.
f) He'll instantly cast it if the player gains, through skills or spells, immunity, reflection, or absorbtion of any kind of skill except criticals (resistance is ok, apparently).
g) He casts it on his next turn when three of his buddies have been killed (see below for the catch to this).
2) He has backup. And he summons another guy whenever you kill one of his backup. His backup consists of 2 demons that constantly debuff you and add damage.
Now, why would you want to kill them when it serves to add the attack of doom and he summons another guy? Because, if one of the demons serving as back-up gets 30 turns without dying, that demon will FULLY HEAL the bastard.
3) He has 27000 HP. And most attacks only do damage in the hundreds. So prepare for a long, long match. [Technically, he has 18000 HP, but after dropping to 9000, one of his buddy demons will heal him back to 18000 via script; only happens once this way, so 27000]
4) He crits. A lot. And by game mechanics, if one side crits or exploits a weak point, then that side gets an extra turn. Oh yeah, and pretty much all his attacks hit everyone for about 300 and even more if he gets in crits. Add in that everyone has only 999 HP at most with his backup adding in damage AND he gets 2 turns everytime his side gets to attack [and possibly more if he hits weak points or crits].
5) The ONLY way to hurt him is to use Earth elemental skills and Almighty skills. Technically, it's possible to hurt him using Guns in the characters' human forms, but that's a death-wish, since the damage is small and it's not possible to use skills. Anything else, he'll resist and you lose turns to attack and heal. This means you're pretty shafted in terms of attacks. Good news in this: Almighty attacks are generally the best damage attacks.
Ungodly hard. 'nuff said.