Immediately Lock-On to Experiment 221 as soon as the battle begins. The boss will dash around the battlefield at a moderate speed, and while the player's first instinct to is to chase after 221, this is not advised, as the extraterrestrial will proceed to charge at Terra at a high speed, dealing Lightning damage if its attack connects. Either keep ahead of the Experiment or Slide to avoid this attack. Land a quick aerial combo, and immediately back away. Once 221 lands, it will surround itself with a damaging field of electricity. Run away until the attack stops, and immediately attempt another combo. As 221's HP diminishes, its electric attack increases in range, but it can still be avoided by Slide or a simple run.
When the player should begin to worry is when Experiment 221 enters the blasters hanging from the ceiling. It will cause the guns to fire, quickly depleting Terra's HP. Land aerial combos to deplete the guns' HP, and 221 will be released. It should be noted that until this time, the blasters will follow Terra with their fire, and it is recommended that Slide be used to avoid damage. It should also be noted that if the player prolongs the battle long enough, 221 will leave the blasters itself, causing them to grow immobile once again. At this point, the boss will then shoot balls of electricity in all corners of the battlefield. These can be avoided by staying close to the Experiment. 221 will then enter the blasters again, repeating its attack process. When it summons balls of electricity from the air, they will detonate as soon as they hit the ground, having a long range. Slide can again be relied on here. So long as the player heals regularly, there should be no problems conquering Experiment 221.