I'll try to explain how to evolve in the most understandable way I can.

1. Beat Story mode (Eg. Beat Dark Dialga and endure the long credits)
2. P*** Graduation Exam (Eg. Beat All the Guild Members)
3. Do a few optional missions (missions on the Bulletin Board) And a Mr. Mime will apper next to the Kecleon Shop. He will talk to you about a Scizor who's been frozen for decades in a frozen dungeon. After talking to him, you will be able to rescue Scizor.
4. Do some more optional missions after that, and then talk to Sunflora back at the Guild. She's gab on about how they found the Surrounded Sea again... and you can explore there now. Get to the end (Not that hard) And retrieve the Manaphy egg. Next day it will hatch. You bring it to the Guild and it starts to cry. Now you have to get some Blue Gummies. (Craggy Coast, Kecleon Shop, Kecleon Dungeon Shop) You need 2. The Next day, Manaphy is on the beach, and p***es out due to illness. You must go to the Miracle Sea (Again not that hard) and beat the outlaw Gyrados who's picking on the Phione. They will thank you and give you Phione Dew. Back at the Guild you heal Manaphy, and set it free (Omg so sad >.<)
5. Do a few more optional missions and go to the lowest floor on the Guild. Team Charm will be there, and they'll talk about how Wigglytuff was part of their team, they found a key like 5 years ago... your partner asks if you can go exploring with them. They say yes. Now here comes the hard part. Each ruin in the Cave requires Unown stones, held by that shaped Unown (the chance of them dropping one is 50% but if they're already holding another item forget it. You need to spell ICE, ROCK, STEEL. (Eg. The Regi Bros.) At the end, you fight Regigigas (and 6 Hitmonlee and 6 Bronzong)
6. Do more optional missions, and you'll have a dream about Cresselia (It's Darkai transformed) Tormenting you. Then, he will torment Azurill. You go get Drowzee and get him to help you. You enter his dream and encounter Darkai impostered as Cresselia. Drowzee helps you after the battle and Azurill awakens. That night, Palkia is being tormented by Darkai and is summoned to eliminate you. He brings you to Spacial Rift and you battle him (My method of beating any boss is throwing a Vile Seed at them =]) After you learn about Darkai and the real Cresselia appers. Palkia calms down and you're back to Treasure Town. You go to Dark Crater. (a bit tough) and you battle Darkai and 6 other Pokemon at the end (Bring lots of Reviver Seeds!!!)
7. Go back to the Hidden Land and Temporal Town, Recruit Dialga. Now go to Spacial Rift and recruit Palkia. Now go recruit Darkai (Sky Rift B14F, Mystic Forest 13F, Miracle Sea B3F) Cresselia AND Manaphy join.
8. Woot you can evolve now :D