I completed 3 games, Yellow, mostly Crystal and Y. The first 2 died shortly after beating the Champion so didn't get a chance to complete the pokedex. But I am trying to complete a living dex for Y, I'm at 683 I believe.
I started it shortly after beating AZ. First I went back to each route and caught every Pokémon I needed, as well as extras for their evolutions. Then I leveled them up in Victory Road using Mew and EXP Share to evolve them quickly. This included Volcarona, Braviary, Hydreigon, Mandibuzz. I've actually encountered 3 full odds shinies in Victory Road while doing this. Then I bought X and a 2ds in order to complete all trade evolutions, well all trade evolutions that didn't involve stones that you only get 1 of in the game. Like Moon Stone, Dusk Stone, Sun Stone, Shiny Stone, Thunder Stone, etc. Then I transferred quite a few Pokémon from Black 2, ie Clefable and Sunflora which took stones I only had 1 or 2 of. I also Wonder Traded at this time, was able to get Wurmple, Golem and Slakoth.
After that I discovered Friend Safari and was able to catch about 30-40 Pokémon I could get there.
Then I discovered GTS, and with Level 1 Cleffa's in Ultra Balls I was able to get pretty much any non legendary/non fully evolved starter in the game I still needed, even Gallade, Mismagius, Nidoking, Nidoqueen, Togekiss, Uxie and Azelf.... However I decided I only wanted to GTS Pokémon that needed a stone that wasn't buyable in the game. That means I didn't GTS Kingdra, Electivire, Porygon2, Porygon-Z, Gliscor, Magmortar or Weavile but instead saved up BP for them. And I evolved all friendship evolutions by riding my bike back and forth with them in my party.
Right now aside from legendary Pokémon I only need Clauncher, Clawitzer and Magmortar. I plan to fish for Clauncher in X when I get there and catch the legendaries in Omega Ruby, though I'm not sure about Kyogre/Latias.