The achievement of the same name in Sonic Generations? Surprisingly overrated. If someone like me can do it, there's no reason why you can't. My successful run was incredibly tense, not just because I had to stay perfect the whole way through, but more importantly because a few HOMING SHOTS nailed me right in the noggin as I was inside the Time Eater's core for the second hit. I think I might've wrecked something had I been screwed out of the achievement because of that. But I didn't, and thus I successfully 100%ed the game. Nowhere near as insanely hard as Sonic Adventure 2, but 100% is 100%.
My big complaint with the fight (aside from the supporting cast NEVER SHUTTING THE HELL UP) is that the whole gimmick of the two Sonics fighting together is utterly wasted. It's always better to stay with Modern Sonic the whole way through, except for the laser attack that immediately follows the first time time gets slowed (probably the best tip I could give anyone going for the achievement). Highly unfortunate that Classic Sonic is wasted like this, couldn't they have come up with more reasons for switching?
But overall, amazing game. I don't think they could've pulled off what they were going for any better. Their recreations of old stages was masterfully done - making brand new stages that look like the old ones, but throwing in some chunks of them anyway. It's awesome how they make you go "hey, I know this". And I'm not just talking about the more memorable moments like running down that building in Speed Highway or the crumbling spiral pathway from Sky Sanctuary, but even minor bits like that staircase in City Escape act 2 right before the truck chase. Best Sonic game? I dunno, I'm a total sucker for S3&K and Colors, but they definitely don't outclass it either. Maybe the best way to decide is to give it, well, time.