Both.
Keep in mind that it is hard to read because you have written the stat block as a PC rather than a monster. I would rewrite it using a standard 5e monster block.
His defensive CR is a mess. His hit points are CR 1 and AC is off the chart. His combined defensive CR is 6.
His offensive CR is also a mess. His dmg/round from what I can tell is 72 which is CR 11. His attack bonus is CR 17 for a combined total of CR 12.
That puts him at an overall CR of 9.
He should either be designed as a monster or a PC. He should have a level or a CR. Not both. 5e isn't designed like that.
This is definitely the standard 5E statblock, 16th level monks just get lots of stuff. The DMG totally tells you that you can make an NPC the same way you would a PC (with a race and class levels) on page 92 so I'm not sure where you draw the distinction between monster and NPC, but I'm game to hear it!
I'm going to assume you are referring to the table on page 274 of the DMG with your totals and while that's not a bad place to look (it's the logical place to look, even!) I gotta tell you, I've found that table is not very reliable. But let's look at that on an individual basis.
Hit Points: He does indeed only have CR 1 hit points. Some other things that have CR 1 hit points -- Imp (10 hp), brass dragon (16 hp), dwarf (26 hp), harpy (38 hp), and the closest we get (aka the highest HP total for a CR 1 creature) is the kuo-toa whip (65 hp). But there's a LOT of disparity here -- displacer beasts (CR 3) have 85 hit points, ettin (CR 4) and galeb dur (CR 6) have 85 hit points, githzerai zerth (CR 6) have 84 hit points, and the real kicker, the demilich (CR 18) has 80 hit points.
I think we can both agree the demilich is probably not a CR 1 creature, amirite?
So there we've got rules precedent for special circumstances regarding hit points. Akuma here ignores the first 3 points of damage from anything, gets a +12 bonus on a save (DC = damage dealt) whenever he gets put down to instead be dropped to 1 hit point, evasion, deflect missile, and he's got that wicked "off the charts" AC. Also, amazing saving throws (which unfortunately the table does not account for, but worth noting I think).
AC: His AC
is off the chart! I even went back and popped it up some to compensate for his low hit point total, to the highest amount I've seen in D&D 5e statblocks: 22 (ancient dragons and empyrean). It's not just what it is because I felt like 22 was a good number, it's a combination of Dexterity, Wisdom, his magic item, and martial arts stances (a special kind of feat in
Mists of Akuma) that makes it so.
Again though, the chart just really isn't holding up to its own standards. 19 of the monsters in the MM have off the charts AC, including but not limited to the CR 4 Helmed Horror (which is rocking only 60 hit points).
Offense: I've got the Ancient Black Dragon open so let's look at this guy: bite for 28, two claws for 15 each, then legendary actions for a tail dealing 17 and some wings for another 15 = 80 damage. That means our dragon is a CR 12 as well, right? But nay! It's CR
21 and should be dealing at least 40 more points of damage a turn to match the chart! Let's check another CR 16 creature and see how it does -- the adult silver dragon rocks out with a bite (19), two claws (15 each), a tail (17), and wings (15) for 81 damage a turn -- a total right around Akuma's! Iron golems are woefully under, only putting out around 46, planetars fare better with 86, mariliths up the game with 93, adult blue dragons look like they're in the boat with the planetars, but of ALL these CR 16 monsters, not a single one actually deals the 99-104 damage suggested on that chart.
I think I can get on board upping his strength a bit so Akuma deals more damage and hits harder, but what do you think with all this tallied here? He's got that knockout or 10d10 special move, can make stunning strikes in his flurries, and will get lots of chances to score critical hits (upping his output by another 9 damage for that turn--so mariliths are probably expecting a critical which *would* bring them up to the chart's value). Is he due a Strength score of 20 perhaps? I was saving that for Zangief/M. Bison but Akuma's badass enough to merit it.
