Your next mission is that you are told the metalheads are moving through the desert. It's time for you to take them out. A new vehicle is unlocked for you - now you get the Sand Shark.
There are four enemies to track down - all are located to the right. Drive over and attack each one from behind. It's not really hard, it's more of a training mission on doing vehicle attacks.
A hologram video tells you that someone is chasing after dark eco crystals.
When you're done, return to the city. Go to the marked spot and take an elevator to the throne room. You have to do another arena battle, and you get a Wave Concussor. Use the up-Dpad to select this. Hold R1 to charge and release a bigger fire blast.
Head back to the arena and down to the glowy spot. When you're done, the king is impressed with you. You get your second battle amulet and a new weapon mod - the beam reflexor.
Jak and Daxter 3 Walkthrough