After you make it through the Cave of the Past, you and your friends face Giygas.
In the first phase of the battle, Giygas is invulnerable, but Heavily Armed Pokey can take damage, so direct your attacks at Pokey.
Giygas will use PSI Rockin α and PSI Rockin β.
Heavily Armed Pokey is vulnerable to Brainshock, so use that on him to make him feel strange. Even if his attacks hit Giygas, the deflection from Giygas's shield will only be 50% as strong as it would have been if Pokey attacked you directly.
Attack Heavily Armed Pokey with Freeze, Multi Bottle Rockets, and the Heavy Bazooka. There is a 50% chance that Paralysis will work on Heavily Armed Pokey.
After taking a lot of damage, Pokey will turn off the Devil's Machine.
In phase 2, Giygas is weak to Brainshock, so have Poo use it. Giygas will cast a few incomprehensible attacks. These attacks are actually PSI Thunder β, PSI Freeze β, and PSI Flash Ω.
You can use the Heavy Bazooka on Giygas, but don't use Multi bottle rockets, because they will not be able to hit him. PSI Rockin Ω might miss, but it is still Ness's best attack.
After Giygas's HP is low enough, Pokey reappears. This means that you are almost done. Your attacks will not hurt Giygas now. The only thing that can hurt Giygas now is Paula's Pray command. Use Brainshock to make Giygas's attacks less likely to hit you, and keep Paula alive to Pray nine times. After the ninth prayer, Giygas will die.
Congratulations! You won! There is more to do before the ending credits roll...
Say Yes when Paula asks you to escort her home. (You can say No, but she will join you anyway.)
Then you can open the gift boxes that appeared. Use Tony's letter on Jeff.
Take Paula home to Twoson whenever you wish.
You can teleport anywhere and explore the world, which no longer has any monsters in it, with the possible exception of a Shattered Man in the hieroglyph room of the Summers Museum, if you avoided fighting it earlier. Make sure not to let that Shattered Man defeat you if you fight it. Otherwise you will be trapped in a Game Over loop.
Many people throughout the world have new things to say, so feel free to take your time to visit them.
You can talk to the Tenda village chief to get the shyness book, then return it to the Onett library by using it on the woman at the front desk.
You can buy the house in the west part of Onett, if you haven't already, and get your picture taken in front of the couch inside.
You can check the Photo Locations page to get any photos taken that you might have missed.
When you are done exploring, after you have taken Paula home, go home and talk to your mother. When you are ready, talk to her to look at the photo album together. The end credits will roll. After that, there will be a cutscene. Congratulations!