pokemon

Well the ones you can catch without too much hassle are rotom, spiritomb heatran, regigigas, cresselia and giratina. They are all part of the game and all you have to do to catch them is be in the right place at the right time... And capture them in a pokeball of course. Spiritomb likely takes the most work to catch, but can be done without anything too special.
As for Phione, manaphy, shaymin, arceus, deoxys and darkrai you will only be able to get them through a nintendo event, such as the coming out of a movie etc, etc or cheating by using an AR. Events seem to come rarely and are only in certain areas, so if you want some of those guys really bad get an AR and cheat. In my opinion it is not really worth it.

I will try and see if I can help you out. You can catch the Pokemon Rotom. In order to catch him, you need to go into the Haunted mansion in Eterna Forst at night, and talk to the Television that is in a room on the second floor. Rotom will pop out of the television if you are there at the correct time, and you can catch him. You can catch Heatran, but only after you obtain the National Pokedex from Professor Rowan by seeing, or catching all 150 Pokemon in the Sinnoh Region. You can find him in the deepest room of Stark mountain which is on Battle Island.

You can Catch Regigigas. You can only catch him after you obtain the National Pokedex, and migrate all three of the regi's(Regi ice, Regirock, and Registeel) to Pokemon Diamond from a previous game such as Pokemon Emerald, Pokemon Ruby, or Pokemon Sapphire. He will be in the Temple in Snowpoint City.

You can also catch Giratina. Like the previous two Pokemon you must have the national pokedex in order to catch him, or to even go to the area where he is. You can find him inside Turnback cave, which is hidden away inside the Spring path. Once you get the National Pokedex, head south of Hearthome city, and you will see a path that you have never seen before...that is the entrance to the spring path.

You can also catch Cressalia. I do not think that you need the National Pokedex, but I cannot fully remember. You can find Cressalia on Fullmoon Island, which is a short sail away from Canalave city. After you beat the Elite four, go talk to the sailor, and his sick son. The sailor should then let you choose whether or not you want to go to Fullmoon Island. You can only get the Pokemon Phione, Manaphy, Darkrai, Shaymin, Deoxys, and Arceus through one of to ways. You can either cheat by using an Action Replay Device, or you can go to a Nintendo Event. Oh...you could also trade someone who has the Pokemon that you want. Those are your options. You can get Spiritomb by getting the odd Keystone, and meeting a friend Thrty-two times underground, the Underground Hiker does not count. Once you have met a friend 32 times in the Sinnoh Underground, go to the Hollow tower, a ways away from Solaceon Town, and use the odd keystone on the tower, spiritomb will come out.

Also yes, before you can migrate any Pokemon from any game, you must have the National Pokedex.

You can catch these Pokemon and this is how for each pokemon.
Note* All these Pokemon you are wanting to obtain is after you either get the national Dex or beating the Pokemon league.

You can catch Rotom by seeing all 150 Pokemon (Manaphy is not included) then once you obtained the national dex from professor Rowan then you go to Enterna Forset haunted mansion and you go around until you see this tv in a small room on the top floor and then you click on the tv and rotom will appear.

You can catch Heatran by going to Stark Mountain at the Battle area. Giratina is quite hard to obtain you must go to Turnback Cave (Near Route 214 then you enter a place called Spring Path which leads to Turnback Cave) Also it's hard to Giratina in there.

Cresselia is like Mespirit but first you go to Canalave then you talk to the sailor and he should say that he wants to go to this island and say yes. Then you are off to the small island where cresselia is and you talk to it then it disappears off like what Mespirit does. Good luck.

You only can obtain Manaphy through an Nintendo Event or Obtaining it through the NDS game "pokemon ranger". You also can breed manaphy and makes an egg and when it hatches it is a phione.

Darkrai, Arceus & Shaymin only can be obtained through a nintendo event. To obtain Deoxys must be from your GBA game. Last but not least spiritomb is only be able to be obtained by meeting 50 people in the underground and then going to hallowed tower south of solaceon town and clicking on the small hallow tower and spiritomb will appear. I hoped this has helped you heaps and you will be fine.

~Twilight_Sasuke~