1. Collect the seven Chaos Emeralds: These are scattered throughout the game's levels, and you need all of them to reach the final battle.
2. Face Dark Gaia at the end of the game: After collecting all seven Chaos Emeralds, you'll face Dark Gaia in its true form, a massive, multi-headed beast.
3. Use the Chaos Emeralds' power: You'll use the combined power of the Emeralds to purify Dark Gaia. This involves using the "Super Sonic" form, which is unlocked by collecting all seven Chaos Emeralds.
4. Perform the purification: While in Super Sonic form, you'll unleash a powerful attack on Dark Gaia. This attack doesn't kill Dark Gaia but instead purifies it, restoring balance to the world and returning it to its original form.
The game's story focuses on the idea of restoration rather than outright killing. Dark Gaia, though destructive, is ultimately a part of the world's balance, and the goal is to return it to its rightful place.