If you enter your name as 'allgrades' (without the quotes) you will begin the game with ALL the upgrades in the game.

When you are in the Normal or higher difficulty setting in 2-Player mode and are dead or almost down to no lives press 'Start' and choose to leave the game. Then press Square to load your character and you will be back at full lives but unfortunately you will have lost everything obtained since your last save such as money, upgrades, etc.

Go to the Imperial Palace level and towards the end is a room full of red beasts and also the Magic key which you need to find. The red beasts can be killed easily by going to the center of the room and repeatedly using the Path Of Sky attack (press X). Keep killing the beasts as they keep appearing and you will continue to gain Experience.