I was significantly under-leveled when I got to the Elite Four, and I don't often have the time/patience for level-grinding. However, having seen all Pokémon in the regional Pokédex except Garchomp, I managed to make it to Cynthia's Garchomp after 3 or 4 attempts at the Elite Four. Then, frustrated, I used Pal Park to migrate in 6 Pokémon from my FireRed that were all between level 65 and 75, added a couple of them to my team, and totally creamed the Elite Four. I felt that I had achieved my revenge. :)