It was a little easier than expected. From what I already had lying around in Bank, X, White 2, HeartGold and Platinum, the main thing I had to do was get legendaries, which required me to play through Ruby again to get the golems, Groudon, Latios and Rayquaza. I already had the birds, Ho-oh and the gerbils, got Mewtwo and Lugia in HeartGold, Conveniently, Kyogre and Latias were in HeartGold, so Ruby got me the complete set of Hoenn legendaries. I'd played Platinum to the end and therefore had access to Dialga, Palkia and Giratina, while Heatran, the Lake Redundancies, Regigigas and Cresselia were available in White 2. I had Zekrom, Reshiram, and Kyurem already because the fusion concept was one I had to see in action at least once, and I had the genies already from the Dream Radar, and so the last thing was the Musketeers, who were waiting for me in White 2. BW2 is really kind of ridiculous with how much is available. Besides starters and a couple of bizarre outliers, I probably could have done almost everything with just W2 and X.
What was most difficult for me to get were Vullaby/Mandibuzz, because I played White and White 2 and it was Black/Black 2 exclusive, and I didn't have a friend safari with it, and... wait for it... Glameow. Glameow is ****ing insane. It was version-exclusive to Pearl, with its Diamond counterpart being Stunky. Neither was in Platinum. Neither was in HeartGold or SoulSilver. Both were available in the Dream World for gen 5 but the dream is dead. Stunky was available in Hidden Grottoes in B2 while the same was true for Glameow in W2... briefly. It was some kind of event. Stunky was in X and Y, and Glameow finally sees the light of day again in ORAS, which I don't have, but you can't even get Glameow on GTS with X and Y alone because you never see one in-game. Holy crap. Nothing else has absences this long except for legends and myths. If even. I got Vullaby and Glameow by changing my profile message in X to fish for trades. Not before some idiot read "Glameow please?:)" and took it to mean "this guy wants a Volcanion egg!" No, I don't want illegitimate eggs containing unreleased mythical pokémon, thanks, also suck my combusken