Play as Vezon:
Successfully complete the game to unlock Vezon.
Defeating Guhhrahk:
The best Toa for defeating Gurrahk level 1 are Matoro for beating Guhhrahk Hajli and Jaller for killing summoned enemies.
Defeating Vezon:
After defeating all Piraka, you will have to fight Vezon for the last time. First hit him in the back, then in the legs, then the heads, then Vezon.