Many good pokemon are tri-evolution(evolve 3 times)
such as Garchomp (dragon-ground) which is found in hidden cave under cycling road/ the cave next to it, it's obtainable before challenging elite 4.
Garchomp
-has good attack
-good HP
-learns dragon claw
-learns dig
Another good pokemon is Gyarados, which evolves from a magicarp(found in water using a rod) at level 20 (Gyrados is also found rarely in most water).
Gyarados
-has good attack
-good defense
-learns ice fang
-learns bite
From experience - I beat the elite 4 with these 2 pokemon and the aid of staraptor(which is relatively good with high attack and speed but low defenses and HP.)