Hey, McG! This sounds really good to me, man. Tons of tension, which certainly fits the Boss Battle theme. A bit more on the creepy side of things, which brings to mind such titles as Parasite Eve and Resident Evil. I'm not sure if you've already adjusted the distortion on that bass or not, but the amount of fuzziness sounds good to me.
Lots of subtle stuff going on within the mix, and your percussive elements are quite effective at creating the right kind of atmosphere. While there may not be a ton of melodic variance, you've broken things up every so often with different sorts of transitions, which helps to both ramp up and maintain the overall level of tension.
I'd be curious to know just what kind of boss is actually being battled here, but since it's an ongoing VG project you probably can't say. The music brings to mind something dangerous and dire; a true threat that needs to be vanquished. The previous suggestion of a Lich doesn't seem too far off the mark, especially since I was personally thinking it would work well for some kind of powerful undead fiend, too. That said, it could be for just about anything or anyone who's gone full dark, no stars.
It's always great to hear new music from you, Dan! May the RPG music gig be kind to you, and I look forward to hearing more. Cheers!