Um yes I was wondeering how to get giratina. Is it an event Pokemon or..

Got ot route 214 on that route a new path will open called spring path simply walk through and when you get to turnback cave go in and theres Girantia. P.s. All the legendaries you mentioned minus crescellia are event only pokemon.

Darkrai, Shaymin, and Arceus are event pokemon.
Giratina is not.
Along the middle of route 214, a new area will open up to your right. Go in the little forest thing, and use rock climb on the wall past a grassy area where you can find level 50+ pokemon, and go around and use rock climb again down the hill. Go in the cave. Venture through the cave. I can't give specific directions because it's all random. Start at any door from where you are. If you're lucky, you'll end up in the room with Giratina in it after you've passed 3 pillars.

*Note* After you've beaten/caught Giratina, try to get to his room again starting from the beginning. You'll either find a stardust, rare bone, or reaper clothe from what I've experienced, in his place.

So I'm guessing you have the national dex, so use your map look at pastoria city then look at the lake above it then move your square right about 2 or 3 times and you'll find something called spring path he's there only you have to go into turnback cave PS you'll need surf rock smash and rock climb.

The event Pokemon are Darkrai,Shaymin,and Arceus.They can only be able to get by a nintendo event or by using action reaplay.Giratina is not an event pokemon.

To obtain giratina you must go to route 214 on the eastern side and have a pokemon that knows rock climb and defog, there should be a new path but it will only open up after you get the national dex.If you go through that new path you should be at spring path.Keep going untill you see a forest opening.After that you will be at some grass.Go north east and you should see a cliff to use rock climb,use it.You can take right or left but the right side has less grass.After you go half way around you will see another rock climb but going down,use it and enter the cave.At this point you should use defog.You will now be at Turn Back cave.This cave is completley random from what I have been through.There will be a message on the wall but ignore it.There are 4 doors 1 on each side. Take any of this doors since its random.In each of the following rooms,there will be either one of three things inside of it:a room with some rocks that you can use Rock Smash on, a room with a lone pillar that has numbers printed on it if you examine it, and the room with Giratina.Going back into the door that you just came from will lead you back to the entrance,so if you get lost,just go back the way you came.The entire cave is pretty much random so it took me a long time to figure out.The farther you get the high level pokemon appear.

I hope this helped you!