going to the question "how do you get oak's letter in Pokemon diamond" to see the jokker and say to it if I kill it
Via Nintendo Wifi.
You have to get oak's letter at on a special event.
There is a road behind the Pokemon league where Shaymin can be found if you get Oak's letter.
Normally, Pokemon Ranger isn't compatible with Pokemon Diamond/Pearl, but if you complete the Manaphy Egg Mission in Pokemon Ranger, you will be able to send a letter to your character in Pokemon Diamond/Pearl asking them if they'd like a Manaphy Egg. In response to your question, you need two DS's and DS Wireless Communications to send the Manaphy Egg to Pokemon Diamond and Pearl, however. No cables will be required, so don't worry about that. Other than that, that's the only way Pokemon Ranger and Pokemon Diamond/Pearl can communicate. If you need help unlocking and completing the Manaphy Egg Mission, research it on the Internet. Good luck.
no you have to get is from an action replay or go to a Pokemon event
Nothing unless you have Oak's Letter.
You use it at the smooth rock. Then you can get shaymin.It is on route 224.
you need oak's letter from Nintendo event [which already past]
route 224
You have to get it from an event, but there hasn't been any event lately for Pokemon diamond/pearl. Most of the events are for platinum now, oaks letter was given out a feew weeks ago for platinum. You can keep a look out though by going to serebii.net
Unknown(sorry about my spelling) Letter A is the rarest Unknown in all games. You are more likely to find a shiny Pokemon then Unknown A but if you are dedicated then go for it but prepare for frustration.
either by cheating or a wifi event (but wifi event expired)