After completing the introductory level where Zero gets his Sword and Pistol, get on the teleporter and teleport back to the base. Immediately exit the place by trying to climb the ladder you are in front of. Ciel will talk to you, and give you several options to select. Keep selecting the first choice she gives you on each option and you will be teleported to a junkyard level. In the middle of the level there will be these pillars that will keep growing back after each segment is destroyed. You can keep destroying the segments with the Sword and Pistol and they will level-up. Keep destroying the segments until the Sword and Pistol are at their maximum levels.

To maximize the skill level of all the weapons, use them at the enemies at the disposal center; at the part where two spiked things spring up and shoot energy bolts horizontally.

Go back to the Underground Laboratory. Climb up the steep drop where you originally defeated Aztec Falcon. Keep going left until you reach the little white anthill-like enemy that spits out five Spider Bots at a time at you. Keep shooting the Spider Bots as they appear until you get the upgrade for your Buster Shot. Do not destroy the little white anthill yet. Also, pick up the small Energy Crystals that they drop. When you finally get the third upgrade (faster charging) for your Buster Shot, you can really start building up your weapons and Energy Crystals. At that point, do not shoot the Spider Bots anymore. Instead, slash them with the Z-Saber, then fire the upgraded charged Buster Shot, dash after it, and immediately start charging your Buster Shot again. As you dash after the fired Buster Shot, you will notice that you hit the Blue Foot Soldier just a second away. Slash him, then fire off the Buster Shot again to the left. Immediately start charging again. Dash after the second Buster Shot and you will hit another Blue Foot Soldier. Dash past him (Dash/Slash him on the way), then stop and face to the right. Fire the Buster Shot again to the right and hit the Blue Foot Soldier. Dash back to the anthill. Slash the Spider Bots and fire the Charged Buster Shot to the right this time. You will destroy the anthill and hit the Blue Foot Soldier on the right side of the anthill. Dash/Slash him. Dash to the right one more time and just regular Slash the next Blue Foot Soldier. Then, repeat the entire process again. Although this process seems long and tedious, it actually takes only less than five seconds to do each time. The constant dashing and slashing will ensure that you pick up anything that these enemies drop, including an extra life that appears after ten minutes. You can acquire about two thousand Energy Crystals after twenty minutes.

After defeating Aztek Falcon and returning to the resistance base, do the "Retrieve Data" mission there you will find balls that have electricity coming out of the sides and are never destroyed. You have to stay close enough to hit it with the Z-Sabre, but for the Z-Buster you can stay far back while shooting it.