Defeat Guhhrahk to unlock him.
Successfully complete the game to unlock Vezon.
Successfully complete the game with all six Inika to unlock Takua Nuva.
Successfully complete the game 100% to unlock a hidden cutscene.
Collect all the Lego bits in the Matoron Enclave, then go to the Priaka Playground, and go back out. Keep doing this for a lot of Lego bits.
Completely upgrade Toa Nuparu at the Matoran Enclave shop. Collect 500,000 Lego pieces and buy "Bonus Level 1". Go to the Piraka Playground. Go to the other side of the playground. Once there, you should find a cave/pathway between the Piraka VIP lounge and the Piraka diving board. Go inside the cave; there will be three portals. Go through the first portal on your left. Then, switch to Toa Nuparu and start destroying multiple enemies while collecting Lego pieces. Do this quickly, as you have a 112 second time limit. You can get an average of 50,000 to 70,000 Lego pieces every time you try the bonus level.
The best Toa for defeating Gurrahk (level 1) are Matoro for beating Guhhrahk Hajli and Jaller for killing summoned enemies.
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.