To access the secret boss:
At the title screen press U,D,D,D,L,R then start & selectat the same time.

Get last weapon:
In Edge Master Mode, after you beat Soul Edge, save your game, go to Italy 
and win the battle. You should get it after that.

There is alternative Arcade mode ending.
Mina: press UP DOWN when talk to her father.
Whang: press B when get close and going to pick up Soul Edge.
Servantes: Press B when he is suffering headache.
Rock: Press B when he is pick up SoulEdge
Li Long: Press rapidly -any button except defence, until he get to SoulEdge.
Miturugi: just dodge bullets by left right and kick him with any attack button.
Voldo: When he is tendering SoulEdge press Up and Down rapidly, this 
ending is bit funny.
Sophitia: Just press up when she is walking in forest.
Taki: Press B when SoulEdge is flying to her.

To play swim suit Sophitia get all 70 extra weapons in edge master. Get all 
Sophitia's weapons to get casual wear Sophitia. Get all Seigfrieds weapons 
to get evil Seigfred. To play as Han Myong use Seung Mina in arcade mode 
and win it without losing a single game. And then when they are talking and 
Han Myong swings the blade down at Seung Mina dodge using the "Down Up" 
dodge.