So the
Sonic The Fighters Achievements are now out there!
http://exophase.com/game/sonic-the-fighters-xbox/achievements/
As with Virtua Fighter 2 and Fighting Vipers, they've made the achievements stupidly easy, even to a point of being a bit lame.
Effectively they want you to play the single player mode through once, and for the rest they use achievements to teach you secret little things in the game. Like in VF2 the achievement is for "Play Ranking Mode in Arcade Mode (hold down P+K+D+START during Attract Mode)" or "Use Dural in Arcade Mode or Offline Versus (In Ver.2.1 hold START when selecting a character)"
So on Sonic's side it's just to finish the game and do some simple things like break a barrier or don't take damage for a round, but it also has:
Honey the Cat
Use Honey the Cat in Arcade Mode or Offline Versus (Press START on Amy and select)