Okay, the title is self explanatory, this will help you obtain all of the legendary Pokémon in this game and I will tell you where to find them. I hope that you will find this useful!

1. Shaymin (land form): 
Level: 30
How to get it: You need to beat the 4th gym and then go to Charpos island and investigate team magma's hideout, Then return to the platepics research center and talk to Mr. Stone and explore the polar forest which is near route A and then celebi will teleport you to the torn world where you can encounter Shaymin.

2. Shaymin (sky form): 
Level: N/A
How to get it: You have to beat the elite 4 and visit Mrs. Flash's house in Mystic town, go into the flower garden, find the green shard and exchange for the Glacidea and evolve Shaymin land form.

3. Latias/ Latios
Level: 40
How to get them: Beat the elite 4 and choose red for latias or blue for Latios when your mom asks you and then they will roam the world map.

4. Celebi
Level: 30
How to get it: After you beat the first elite 4, examine Celebi's statue in the bee bug forest and then Celebi will appear. Then examine another statue in the forest and Celebi will be in your party.

5. Giratina
Level: 55
How to get it: First, you need to beat the elite 4 and then go to the polar forest, go into the deep forest and then enter the torn world.

6. Keldeo
Level: 50
How to get it: Finish the remaining quests or missions to obtain 50 QPs and then exchange them for a rare ticket at the challenge factory and then talk to Mr. April at the Ch. Harbor and then go visit outcast island.

7. Lugia
Level: 70
How to get: Finish the quest to get 50 Qp's and then exchange for mystic ticket and then talk to Mr. April at the Ch. Harbor and then go to Naval rock.

8. Deoxys
Level: 30
How to get it: Finish the quest to get 50 Qp's and then exchange for mystic ticket and then talk to Mr. April at the Ch. Harbor and then go to
Birth Island
 
9. Rayquaza
Level: 70
How to get it: Finish both missions, then examine the book in the secret room in the malias library. Explore team magma's hideout and return to the festa zone and aqua town, then beat archie, awake rayquaza at the top of sky pillar and finish Kyogre- Groudon event. You can come back to visit this location to get Rayquaza.

10. Cressellia
Level: 70
How to get it: Finish the rayquaza event and then beat the elite 4 again and then use the hm for rock climb, return to Mt. Fullmoon and cresselia will be there.

11. Dialga
Level: 70
How to get it: Finish the rayquaza event and then beat the elite 4 again and then use the hm for rock climb, return to Mt. Fullmoon and dialga will be there.

12. Kyogre
Level: 70
How to get it: Finish the rayquaza event and then beat the elite 4 again and then go to route r and dive under the water.

13. Groudon
Level: 70
How to get it: Finish the rayquaza event and then beat the elite 4 again and then go to Path A (challenge islands) and enter Terra cave.

14. Mewtwo
Level: 70
How to get it: Finish the quest to get 50 Qp's and then exchange for the old sea map, then talk to Mr. April and visit abandoned island

15. Entei
Level: 40
How to get it: When you can use rock climb, go to ancient ruins which is north of golden city from the braile wall, down 2, left 2 and then use rock smash and then it will appear in the route K cave.

16. Raikou
Level 40
How to get it:  When you can use rock climb, go to ancient ruins which is north of golden city from the braile wall, unlock the bottom right cave and go around the cave in a clockwise motion around the edges and then it will appear in the altering cave which requires rock climb

17. Suicune
Level: 40
How to get it:  When you can use rock climb, go to ancient ruins which is north of golden city from the braile wall, unlock the middle right cave which you have to use flash on and then it will appear in the route U cave which requires rock climb.

18. Ho-oh
Level: 70
How to get it: after you catch the trio of legendary dogs, go back to the ancient ruins north of golden city, unlock the top cave, use dig to enter the basement, put wailord first and relicanth last in your party and then upgrade the app for the s machine, go outside and a secret entrance on the top left will open where you can catch Ho-oh.

19. Victini
Level: 70
How to get it: After yo get the unown app for  the S machine, talk to the man in the ancient ruins and obtain the unknown letter, go to the battle frontier and deliver it to the man (you need waterfall) and then you will teleport to Cryptic City and challenge Mr. Victor and if you win, you have a chance to catch Victini.

20: Mew
Level: 30
How to get it: After you get the unown app for the s machine, go to Artisan hill (route Y) and Mew lives on the top of the hill

21: Jirachi
Level: 70
How to get it: After you can use rock climb, visit A area and talk to the girl you rescued (mission 1) you will obtain the wish stone



Well, I hope that this helps you catch all of the awesome legendary Pokémon from this game. If you need anything, have questions, comments or concerns, feel free to leave them below. I hope you can use this to help you on your journey, happy gaming and legendary hunting!

wow this walkthrough is great well done razor I really enjoyed reading this

Razor-987 : Amazing Walktrough, this Guide says where you can catch the Legendary's in Flora Sky. < This also helped me a bit.
Thanks for this useful Guide and keep it up making them!

Thanks man. This will come in handy if I ever decide to actually play this game. TO be honest, though, I doubt that I will. But there is always that possibility. 

thing1 : Thanks man, I tried, you never know, you might. And if you do, this will always be here to help you. Thanks for the reply
Eniitan :  
TornadoMudkip :  

well I am glad that you all liked it, I tried to make it as detailed as possible to make it easier to find these Pokémon. It took a while to make this, but I think that it is all worth it though. Glad you guys find it useful and that it is a good walkthrough. Thanks for the comments guys, I will keep making more walkthroughs. I appreciate your positive replies!

Not sure if you knew or not, but I already posted all the legendary locations, in my questions guide over a year ago.

Razor-987 : Thanks man this really helped me.

Razor-987 : i accidentally disliked it as i clicked the wrong one and i cant undo it for some odd reason AND you forgot some legendaries like arceus.