Drops: Skeleton Key (100%), Hydra Charm (50%), Curse of Emptiness (10%)
One of the Core Friends. Technically optional if you defeat at least three Cores to open up the Neo Legion encounter, but fighting Victor is recommended because of the Skeleton Key drop, and also because it will mean that Neo Legion's overall HP is reduced and moveset has a reduced chance of some of the more ragequit-inducing AoE attacks (which it's very fond of spamming).
It's a lot easier to cheese this fight if you use the Assassination Contract with Cathy White here, though bear in mind this will lock you out of Layla Tov's questline and assistance later. Cathy offers powerful ranged support, sometimes landing criticals, which make it a lot easier to connect with long combos without opening yourself up to Victor's lengthy counter-attacks.
Before you begin, make sure you go around to the right of the (now shattered) Arc Du Triomphe (hug the edge of the arena so not to trigger the cutscene) for a chest containing 5x Fortify Athletics. There are three Big Friends here which are dormant but can aggro you during the fight, so it's best to take these out now.
Victor has two attack stances that you need to be aware of. The first, he'll give a horrid laugh and bend over backwards before running around on all fours. At this point, dozens of Friends will spawn in at random around your location. You'll know how to fight hordes of these folks by now, and any strategy you used in the Champs-Élysées will work here too (including dodging the fuck away, because why not). The main difference is you need to focus on chasing and attacking Victor whenever you can, as he will substantially buff the Friends, do long-range Ethics and Maths at you, as well as this really cheap move that will automatically give you the 'toxic' status effect. The latter is telegraphed pretty heavily, though, as Victor breathes in and charges the attack first, so if you can get an interrupt off on him, great (you just better hope the RNG didn't put like 50 Friends in the way, first).
The other stance is obvious, as Victor will bellow, and Friends will rush to him to form a grotesque sort of sedan chair / four-legged abomination. Victor's invincible during this period, but you can attack incoming Friends to weaken this form and make it easier to take down. Anyway, once they're done, Victor will chase you around the arena, trying to crush you. The feet follow a set couple of patterns, so once you learn these, you should be able to dodge them pretty consistently, especially if you use the terrain to your advantage. In this form, you need to focus your attacks on one of the legs where possible, which will eventually cause Victor to fall to the ground where you can damage him. You *really* want to do this, because if you don't take down this form in time, the walking chair will do a very powerful arena wide attack that causes random spikes to appear from the floor. These can easily kill you in one hit with a low-DEF build.
When Victor is on about 20% health (he'll say something like - “We cannot die - we are immortal!”) both of the forms get a couple of extra / more powerful attack patterns, but nothing you shouldn't be able to handle.
— extract from a .txt boss guide for Witchcraft: Shadows over Paris by starlight_m00n