I have to be honest about this one. At first, I was like: nerf it. We can't beat this. I think that's because compared with Unova, Hoenn and Kanto are very very easy to beat. You know the Pokémons every trainer has and its movesets, only choosing attack by the Pokémon type.
Unova is different; they predict you, they switch when they haven't have the advantage, the Pokémons have coverage moves etc. etc.
I think that's the main issue with Unova. And I finally beat it, with some time.
Elite 4 was not that hard. It was hard, but it's not too hard. I think the problems are N and Ghetsis. That's where the challenge begins.
N is where it begins. He keeps switching, every type has coverage. And that's the point where you have to think and be a step in front of N. If you have the advantage, he is likely to switch and pull something out that resists your move. And that's what happens all the time. After a few times N, you can predict what he does and easily beat him.
Well, easily. I had to Hyper Potion stall his Reshiram because it did too much lol. Just stall his fire move 8 times, and it's dead after that. Same for Ghetsis' Hydreigon.
The same for Ghetsis; if you can predict N, you can predict Ghetsis too.
I didn't used any competitives. I just bought a Gengar lv 52, with a nature that decreases Sp. Atk lol. Leveling is suggested tho, between 54-56 should do it. Just farm Basculin horde in Victory Road. They give speed EV's, which made some of my Pokes quicker then theirs.