The VGC, Smogon and Battle Spot do not allow illegally obtained or modded pokemon, and I am assuming any other kind of wifi competition will go by the same rules. Having a battle with friends, of course anything goes. I wouldn't risk disqualification by going in with a pokemon that has been tampered with, but many get away with it and thats just sad. There are no good ways of detecting these pokemon either. The recent case with the Aegislash in a Dream Ball, that was caught by people watching the battle take place, and the giveaway for it was so fast and, albeit, arbitrary, I wouldn't blame the mods for not catching it at all. But the internet always watches, always waiting for these kinds of things so they can call it out.
Legendaries come with AT LEAST 3 perfect IVs in the wild, and its a matter of soft resetting to get them higher than that. So it is totally doable.
Most likely Suicune will have to be traded for though, since it is not catchable in XY or ORAS. Diancie is also unobtainable in game and would have to be traded from someone who was at the gamestop event to get it, which I can try and find out from my friends if any of them have it but that is as far as I believe I can do.
Everything else can be bred and found. Overall price will be 3750pc for the Rotoms, Terrakions, Landorus, Latios, Magnemites and Beldums. Diancie, Suicune and Zapdos will have to be traded for most likely, so I can just do that for free since you are cool and such! Do we have a deal?