I was looking through the code and found this small tidbit about the Phantom Slasher plunge. His battle stats are determined by Rhue's evilness stat. Pretty much, the more evil Rhue has been throughout the game the stronger the Phantom Slasher will be in the battle. Which makes it even more metaphoric since he is fighting his own evils. To be more specific:
HP = 250 + evil*10
Plunge = 18 + evil/2
DT = 75 + evil*3/2
The rest of the stats are constant.
This explains why the PS always went down in like one or two hits for me, I'm usually near max stats and minimal to no evil lol