This is one of the Boss Battle themes I wrote for my Sonic RPG! It plays when you fight Shadow and Rouge on the numorous occasions you do, but it also plays on a couple of other battles(like when you play as Shadow and Rouge and have to fight Sonic's party). I really like the sound of it! It really gives that dramatic sound of fighting the ultimate lifeform.
Synthesized using instruments and samples from all 3 Sonic Advance games(giving it an authentic sound)