In single player mode, repeat the tutorial about fifteen times, then go to Death Valley. Once there, follow a path and fight some Goburimon; do not go across the bridge. Follow the path up and fight the Goburimon. They will give 10 experience points each. Break, then get the Gate Chip, use it, and repeat. Additionally, go to Death Valley, then to the Fortress. When you are inside the guard room, defeat everyone for 32 experience points per kill. After all the doors open, go outside and enter the fortress again. This must be done two or three times to reach the next level.