Before entering the Elite Four get Zapdos, Articuno and Moltres.
Train all of them to around level 60-70.
Also try to get Squirtle as a starter.
It can also come out very handy if you have a Ditto around level 60 because he has the move transform and he will be able to change in the opponent's first pokemon, but be careful, he has only 5pp of each move!
Also get a fire-type pokemon (only if you have chosen squirtle as a starter)for the champions Venusaur.

Then you'll get:
Zapdos
Articuno
Moltres
Blastoise
Ditto
Fire-type pokemon

Also beware that the second time you challenge the Elite Four and the Champion there pokemon are superstrong because they have grown by like 12 levels each!