Play Mission 3: Trial By Fire, and make sure to set the XP bonus using D-pad Right. Proceed through the mission until you reach the battle on the shore where the ships come crashing onto the beach. Try to get the highest possible combo. The higher the combo, the more XP you will earn. When the battle is over and you are told to take out the tower, run towards the archers to kill yourself. You will then start over at the beginning of the battle. Repeat this process until you have obtained the desired amount of XP. It should take approximately ten battles to have enough XP to max out your character. If you set the difficulty to Legendary, you will obtain a lot more XP. You need 250,000 XP to obtain all the locked upgrades.