To unlock all songs & all difficulty levels for Challenge Mode, play the game as normal until you unlock all the hidden songs in the game. Note that this will also list every Challenge song/level as Completed even if you have not yet attempted them.
MAX 300-AAA a song (all Perfect steps, any difficulty, DO NOT USE THE "LITTLE" MODIFIER), or get it on Roulette and pass it (do yourself a favor and do it on Light).
PARANOiA Rebirth: Pass PARANOiA Eternal on every difficulty (DO NOT USE THE "LITTLE" MODIFIER).
Colors (for EXTREME): Play the game/any song 100 times.
ORION.78 (civilization mix): Play the game/any song 300 times.
Put Your Faith In Me Saturday Night Mix (quite possibly THE most horrid song ever on DDR): Fail a song.
Sana Morrette Ne Ente BLT STYLE (THIS. IS. WORTH. IT.): Pass Sana Morrette Ne Ente on every difficulty (DO NOT USE THE "LITTLE" MODIFIER).
MGS2 mission R: Get your first A grade.
Gradiusic Cyber (another): Acquire your first AA grade.
Healing Vision (angelic mix): Pass Healing Vision on every difficulty (DO NOT USE THE "LITTLE" MODIFIER).
Undoubtedly, MAX 300 will be the hardest thing to acquire. Roulette is frustrating as hell, so stick to AAA-ing a very easy song for you. Be calm, and don't rush. Don't let it get to your head if you're getting all Perfect steps and getting close to the end of a song. Just know your heart's beating real fast and that's it.
play 300+(it doesnt matter if they are cleared)
songs to unlock orion 78 from DDR Max 2 arcades