Darkrai, Kyogre, Groudon, Arceus, Dialga. Darkrai beats Giratina, Lugia, Jirachi and some others. Kyogre destroys almost anything, Mewtwo beats Shaymin-S, Darkrai, Blissey and some Arceus. Groudon boosts Fire Blast from Mewtwo and rapes Tyranitar, Jirachi, gives you weather support. Dialga resists Dark, Draco Meteor slays Giratina and Fire Blast owns Jirachi.
As far as I know there is only one code for it, but it can be a bit tricky. What you have to do is keep holding the trigger for the code (select, unless you changed the trigger to something different) while you walk through the doorway, all the way until you're inside. What I do is hold Select until I see the stationary inside. Hope that helps!