The best Toa for defeating Gurrahk (level 1) are Matoro for defeating Guhhrahk Hajli and Jaller for killing summoned enemies.

Unlock the following characters by performing the corresponding tasks:

CHARACTER - HOW TO UNLOCK
Guhhrahk - Defeat Guhhrahk.
Vezon - Complete the game.
Takua Nuva - Complete the game with all six Inika.

After defeating all Piraka, you must fight Vezon one last time. First attack him in the back, then in the legs, then the heads, then Vezon. After that you can play as Vezon in all levels.

Complete the game with 100% completion to unlock a hidden cutscene.

To rack up Lego pieces, fully upgrade Toa Nuparu at the Matoran Enclave shop. Collect 500,000 Lego pieces and purchase the "Bonus Level 1". Go to the Piraka Playground and head to the other side of the playground. Look for a pathway that leads to a cave and enter the cave. Enter the first portal on the left. Switch to Toa Nuparu and start killing enemies while collecting the Lego pieces. You will get about 50,000 Lego pieces.