Kill enemy's until you are close to leveling up. Then, travel to a incomplete data boneyard. Wait until some enemy's spawn, then select the "Level Up" icon on the touchscreen. However, do not click any of the four options displayed on the touchscreen. If you do, this glitch will not work. All the enemy's will be frozen and unable to attack while leveling up. While they are frozen, attack them. When they die, they will not disappear until you finish leveling up. You will know they are dead when you can no longer hit them, and they give you XP. More enemy's should spawn. Keep killing them until there are no more remaining, then finish leveling up by clicking an option on the touchscreen, and the Boss battle will begin. Repeat this as many times as desired.
The Magic Hammer