Go to League Challenge, choose WCW and play the game until you get to Dallas Page. If you defeat Diamond Dallas you can choose him as a playable character.
Play as Glacier
To access Glacier, beat IU in League Challenge mode.
Play as Joe Bruiser and BlackWidow
When you beat all the different game modes, you will get to an extra menu called Whole World Wrestling. After you beat both Heavyweight and Cruiserweight you face a boss. Defeat that boss and you will be able to pick him as a playable character.
Play as Macho Man Randy Savage
To access Macho Man Randy Savage, beat NWO in League Challenge mode.
Play as Wrath
Choose DOA and play until you meet Wrath. Defeat Wrath and you can select him as a playable character. Be sure to save your game from the option menu.
Easy Win
While in the ring go to the left sides ropes and strong grapple to throw the other player out of the ring. Once other player is out of the ring make sure he/she is down then run over to the right side of the ring opposite of the CPU player and get in the ring. Stay on the apron or just stay on the right side of the ring and the dumb CPU will just stand there and get counted out every time.