How do you beat the elite four and your rivil?

Your level need to be kind of high, I beat it with 5 50's and a level 90 from my friend.

My team when I beat it was
Blastoise Lv.64
Lapras Lv.51
Articuno Lv.54
Pidgeot Lv.46
Nidoking Lv.40
Arcanine Lv.37
And alot of revives and restores(I used Nidoking and Arcanie to revive one of the 4 better one because the can't do much)
So teach a Lapras thunder bolt to help bet Lorelei(it will help),Pidgeot and Articuno fro Bruno(but use Lapras for the Onix and teach Lapras Surf and if it's leveled up it should kill it one hit),Starter for the 3rd person,I used Hydro Cannon from the woman and switched every turn so it didn't have to recharge(teach your starter that move and she will be easy),then revived them before Lance and my Blastoise knew Blizzard and killed all Lance's Pokemon on hit, then for the Rival if you got:
Squirtle:use Articuno and teach it Ice Beam to beat the Venysaur he has
Charmander:use Lapras and it's thunder bolt for Blastoise
Bulbasaur:you could use lots of different things but I would prefer Lapras and use a couple thunderbolt's, and the rest a high level Starter could take
My recommend team for you
Starter Lv.55+
Articuno Lv.50+
Lapras Lv.50+
Pidgeot Lv.50+
And any other 2 Pokemon to heal your fainted Pokemon during battle.
I don't know if that will help but that's my suggestion!!

My Team

(mine)Charizard lv78
(friends)ShellShocker(Blastoise)lv95
(mine)Dragonite lv 97
(mine)Pikachu lv89
(mine)Pidgeot lv93
(friends)Weedsprout(Venasuar)lv100
you should train.