Assuming an Omega minus and top tier skill wise assassin. Will differentiate between combat, which is regular, and assassination, where the CA gets a bunch more advantages due to how the encounter is structured.
Dark Souls: Probably dies to tier 4 PCs and SoC, though can definitely get past the rez and should beat all the others. Assasination, solos.
Magic: The Gathering: Really makes a mess, but I don't think he beats
Nicol Bolas either way.
Bloodborne: Should beat everybody either way, as the protag doesn't have the experience and skill of everyone that they kill and as such technically aren't as skilled. Don't you love when the differences between the two systems are nonexistent in game, but matter here?
Berserk (Verse): Should clear
Cthulhu Mythos: Should clear minus 1-A
Darkest Dungeo: Should clear
Destiny:
Oryx probably becomes an equivalent to Abaddon here, but they should kill his
daughters, a lot of other Hive Gods, lots of Vex, all of the Fallen, and many guardians. Assasination wise Oryx is still probably not going to go too well, as even getting one on to the dreadnaught is very hard, but suffice to say that if the PC was a Culexus assassin the raids would be a lot easier than they are.
Warhammer 40,000: Less haxxed dudes do better, as always. An Eversor or Chapter master has a better chance than
Ahzek Ahrima, funnily enough. This is obviously their verse of origin, so just go by canon and say they stop at Abaddon either way.
Salt and Sanctuary: Should clear
Hotline Miami: Should clear
Hollow Knight (Verse): Should clear
The Legend of Zelda: I have no idea, this isn't a verse I'm that knowledgeable on. Maybe I'll argue with Cal about this or something.
Super Smash Bros.: Should clear
DOOM: Should clear