I dunno about final boss theme but it could easily go with an important boss like someone the party has been chasing the whole game and finally gets to fight a few hours before the end or something.
I don't understand what is so good about the solo honestly, it doesn't have the emotion if George Lynch, so why should I even try to like it lol, George Lynch LITERALLY is all I listen too when it comes to instrumentals. Mr. Scary is his most famous, but he has another one called 'Cry Again' that was made around the time of Mr. Scary, but it's just home soloing over a backing track that they couldn't put words too. it's mind blowin.
I believe you're referring to harmonies? Check out Black Star by Yngwie Malmsteen, skip to 1:00 onwards and that's what you're after I believe. Altitudes by Jason Becker also does it in some parts, if that's what you were referring to you can message me and I'll give you a list of loads of songs that do it :)
