If you have emerald and you know how to clone,lucky for you! Just clone any legendary or strong pokemon and migrate them to diamond or pearl. For example, you are playing diamond and you want a palkia, offer a pokemon from emerald and set level as 'any level',this way more people will trade to you. Btw, everyone on GTS wants Mew,Ho-oh,Lugia,Deoxys or Mewtwo.So, good luck!