This Alternate Boss Run Was Unfair... (R1999 Roguelike, Difficulty 18)
14...but not for me. This run was unfair to the boss.
I decided to move up to Difficulty 18 with Insightful Aid and the alternate route just happened to appear on this run, so I went for it. This clear ended up being even more ridiculous than I imagined.
The two Incorporeals I have (both of which I picked up very late in the run btw) were ridiculously good RNG but I only got "hit" twice. Even if my party died both times, the Ancient Chalice had it covered with the two free revives.
The real highlight is the ridiculous amount of control you have over this fight with Insightful Aid. The boss isn't even allowed to get its free Ultimate off at the start of the fight since I can immediately spawn La Source's Ultimate for (checks notes) 10 Stamina. It's pretty ridiculous, it means you never have to worry about its Ultimate while its still in its first phase. You only need to focus on surviving the Debuff Incantation (and there other cheese strategies for this, namely Regulus' Ultimate, which you can use to dodge non-Ultimate attacks).
Assuming you ended up with a good artefact layout for your Aid Incantation nuke, your only worries are running out of Stamina (since you can't get any Stamina back like you can with the Razor fight) and rushing down the boss in its second half (since it becomes immune to Moxie Reduction).