In Pokemon Fire Red, Do i keep battling the Elite four over and over again.how do i get Legendarys without hax?

i have high 50's, mid 60's and mid. 70's as my levels. how many pokemon do i need to catch in order to beat the game? in the Nat. Dex?i have a mewtwo, charizard, lapras, snorlax, erfree, and zapodos as my pokemon, just FYI. THX!!