From TGDB.net

Hints

Fight as Bokuchin:
Defeat Bokuchin during the game.

Ryumaou tournament:
Earn nine medals to play in the Ryumaou tournament.

Fight as Ryumaou:
Successfully complete the Ryumaou tournament.

Easier difficulty level:
Highlight the "Easy" difficulty option, then repeatedly tap
Left until "Very Easy" is displayed.

Harder difficulty level:
Highlight the "Hard" difficulty option, then repeatedly tap
Right until "Very Hard" is displayed.

HINTS:

Disappearing Wiler shirt:
Select Wiler do his Ninja Circus. If done correctly, he will
not be wearing a shirt while climbing down the stick. However,
it will reappear after he gets off the stick.

Mystery third place winner:
Win the championship on circuit under SD Hiryu. Two other people
will appear, but at times the third place winner will not be
anyone that was in the championship.