mewtwo is legendary, by far the most unfair pokemon in the game. find some pokemon with different types. suggestions are sceptile, another eevee evolution of your choice and liking, garchomp, starmie, electivire, heracross, swellow could be good if you could train it correctly with the right item, and dragonite is ok. the site below is a great pokemon site that could help you out with choosing the right pokemon for your play type.
you should make your pokemon adaptable to the elite four, so you dont get wiped out. as you can see, an ice type will make lance, the champion, easy to beat. Lapras maybe?
You should get a grass type pokemon i recommend victrebell or a grass starter you should also have a fire type pokemon and a flying type as well
to the dumbass above mew2 is a legendary look it up
Use Mewtwo. He's not legendary, he just happens to be one of a kind in the game. Like Snorlax or Sudowoodo.