Hi!
I saw in another post that Solaris' theme was missing, since no one could find it. I did some digging for a few hours but sadly didn't find an exact match. What I did find, though, were some similar remixes of Asgore's theme. It's not much, but maybe one of them can serve as a placeholder until the correct one is found? Also added another song that, although not a remix of Asgore, I thought would fit the situations you fight Solaris in.
Asgore remixes:
-First (Starts at 0:30)
-Second (Starts at 25:40)
-Third
-Fourth (Bulk of the song starts at 0:52)
And also Artorias' Theme, I think it fits specially well when facing his level 75 Garchomp at Pyrous
Great taste with Carvanha by the way