It's not too bad honestly. Once you catch all the Pokemon in the dex, minus the event-only Pokemon, you can get the shiny charm, which increases chances of encountering shiny Pokemon by 300%. And the Pokemon Bank makes transferring all 720 across my 6th gen games super quick and easy, so I've got the dex complete on Omega Ruby, Alpha Sapphire, and X and Y. Also having 2 3DS's makes it pretty easy to just farm the same route on both at the same time until I get all the shinies I want, then move onto the next. Lately I've been going for shiny Latios and Latias from the event last year with the Eon ticket. I got Latios shiny, but I'm still working on Latias. Legendaries are definitely the most time consuming.