The Custom Groove option appears once a normal Boss has been defeated (normal Akuma or Rugal). Normal Bosses appear by fulfilling two of the four following conditions.
Achieve over 1000 GPS by the end of the game.
Achieve four or more special KOs.
Middle Boss has appeared.
Achieve two or more LV3 super combo KOs.
Beat up your opponent until his/her life is low enough to finish them off with a special KO. Stand near your opponent and wait for them to taunt you. Then, do the special move and you will get a finest KO. You also have to either beat the mini-
Boss, or receive 1500 GPS before you finish the final round in Osaka to get to Ultimate Rugal.
Hidden Bosses can be used once they have been defeated. The conditions of the Bosses to appear are to fulfill two out of the following three conditions. If you play with a Capcom groove you will get Ultimate Rugal, and if you play with a SNK
groove you will get Shin Akuma.
Achieve over 1500 GPS by the end of the game.
Defeat the Middle Boss (Geese or Bison/Vega).
Achieve at least one Finest KO.