Full list of all 12 Sonic the Fighters achievements worth 400 gamerscore. It takes around 0-0.5 hours to unlock all of the achievements on Xbox 360.
Complete Stage 1 of Arcade Mode (any difficulty)
Complete Stage 2 of Arcade Mode (any difficulty)
Complete Stage 3 of Arcade Mode (any difficulty)
Complete Stage 4 of Arcade Mode (any difficulty)
Complete Stage 5 of Arcade Mode (any difficulty)
Complete Stage 6 of Arcade Mode (any difficulty)
Complete Stage 7 of Arcade Mode (any difficulty)
Win a round without receiving any damage in Arcade Mode (any difficulty)
Destroy all of an opponent's barriers while playing Arcade Mode or Offline Versus
Turn small by being attacked (ie, by Amy's↓+P, etc) in Arcade Mode or Offline Versus
Use Honey the Cat in Arcade Mode or Offline Versus (Press START on Amy and select)
Fight a mirror match in Arcade Mode or Offline Versus