Wow I didn't think it's that much of pain but I guess it will only suck for my 7th gen pokemonUnfortunately, training to Lv100 is a complete pain in Sun & Moon, because there's no Vs. Seeker or reliable places to train. You're pretty much going to have to rely on constant rematches against the Elite Four, or using services like Festival Plaza's Rare Kitchens and Poke Pelago's Isle Evelup.
On top of that, Sun & Moon brings back 5th Gen's horrible scaled EXP system where you only get more EXP against a higher-leveled target. I'd say train everything in your 6th Gen game first to minimize the pain.
I didn't think it could get worse than xy well it's good to know thank youI implore you, friend, please use gen VI for any leveling. Sun and Moon is arguably the most painful set of games for getting level 100 out of all of them. Take gen V's annoying exp. mechanics, plus no high level Pokemon to farm off of other than an elite four with mons chilling at 60 (and not even six Pokemon on their team.) It's not fun at all. Use X/Y and ORAS as much as humanly possible for getting level 100s.
That's sounds good but I don't have oras so I wouldn't really knowOR/AS blissey bases will always be the best way of training to level 100, its just so quick.
The only "best" way to train is by giving a Pokemon a lucky egg, then battle the E4 with it/use an exp share to level it up. This method still takes a while though.