(I call the Manussya stance human stance out of habit, for reference)
You best move is probably Slash of Eternity, since it is one of the stance bombs, so you get bonus damage from the tree. Pascal's Moon Reflecting Satellite slash is not in any stance, so it doesn't get the damage bonus. It may be better than Slash of Eternity, but I wouldn't bet on it.
Depending on how many points you have, you may want to put all your points in offense, so 10 ranks in Strength. At least 16 points in Asura sword for +40 attack.
It's better then SoE. It also has super high accuracy which you need so that Byakuren doesn't have to buff Youmu's. But yeah; it's definitely stronger. The one thing is that is DOES have one turn higher cooldown; you'll be spending the turns buffing up on the first use anyway, but then you'll have to wait slightly longer for the second. In power vs. cooldown, the two might even out, but I'm not sure exactly how much damage SoE does in comparison. I know with proper preparation, Pascal Satellite Slash does almost enough to two-shot the boss (If Dark land is high, it probably -does- twoshot).
Three uses of it will kill the boss for sure if you're got all the proper buffs applied on each, so the power isn't too much of an issue. Use Human stance and make sure to max the Katana tree for critical boost, along with enough points in Half-spirit for the spellcard reduction (although I'm sure you know that already), the criticals are far more powerful then the strength bonus off of Asura. I'm pretty sure Asura's thing only makes the first hit a critical as well, so the buff in Human stance is a lot better as well as it's passive critical boost.
And yes, use the katana, not the Kusanagi. The critical boost in Youmu's tree and the chance to use physical skills (instead of just normal attacks) twice makes it superior, ESPECIALLY the latter if it triggers, because that will cut several turns off the time it takes to end the boss. If one of your spare people in party 2 can use swords and has nothing better to do, they can whack away with the Kusanagi for a little extra damage.
I'd put Alice in the second party. You should definitely have the Autobarrier unless you've got some other shield you think is comparable. Poison Shield is undoubtably the way to go. With Poison Shield and parrar, magic will still be doing a little damage; keep this in mind and give appropriate people resists/mdef to counter, and do have heals on Byakuren (as well as Reimu/Satori if they are present).
Satori/Aya/Mokou are all considerable people for the first party in addition to the ones he said, especially considering using Chasers will screw up chances to doublecast, and that chasers are falling behind in power at this point. Keep in mind the sisters in phase one have some divine barrier; if you've got a staff with Magic Exorcism, use it. The reason I say Aya is she can blinktank, and her evasion spellcard has application versus Anastasis. Whoever the tank is, though, make sure they've got high Elec resist to take Mad Thunder hits. If Satori is in the first party, she can petrify mooks so you don't have to worry about them anymore. (Don't kill the mooks, having a maid summoned sucks.) Lithos has the most divine barrier and the highest mdef as well iirc, so she's the hardest to damage; I'd just take a break when she comes up, especially since her weakness Fire will counteract the water land you build up hitting Biotopos.
Sanae could attempt using a dagger to petrify mooks in her spare time if you've got the Psycho Dagger from stage 13's ghosts. The mambele knife from the Seely Court enemies in stage 20 (I -think- it's them?) has slayers on phase 1 sisters if you want to try using someone (Probably Sakuya) for that. Actually, Patch might do better damage with a Mambele then with her magic, you'd have to test. Sakuya/Mokou bashing phase one with the kusanagi would be alright since it's high power and has exorcism, even if it doesn't slay them, and would help you hit phase 2's damage quota I think if you saved mp for that.
If Alice is not in the first party, keep in mind you want earth resistance on everyone for phase two's Stone Shower.
You can get away with having one of the elements at max from your party using it to kill the second phase, if your first party has Mokou and you just use Reraise to take the nuke before second party jumps in. (Alice also can try to do this for party 1 spamming london doll/little legion until it's used) Light/Dark don't trigger the murdernuke, either, and having high Dark boosts the damage form hitting the dark weakness.
Keep in mind you'll need to be able to pump out enough damage to kill the second phase in time. It's 50,000hp over 9 turns, if I remember right. You might consider using Team 2 for this phase; if you do, keep in mind Youmu will need enough bombs to handle both this -and- the last phase if you go down that road.
(in my current 100%-goal playthrough I'm in stage 14 right now myself, farming for drops I didn't get before I fight Tenshi. Steady progress~)