The leader of the metal heads is the last boss of the game. After you start fighting him, there is no way to go back to the city until you defeat him, which means you can't collect ammo other than the blaster ammo around the nest, so be sure to come with full ammo of all gun barrels. It is also a smart thing to have all the dark Jak abilities, including giant dark Jak. There are three parts to the boss:
1) When the battle starts, the boss lays metal head eggs that dont hatch for a few seconds. Use this time to shoot the metal head leader with a few blasts from the peacemaker or whatever you have ammo for. When the eggs start to hatch, dispose of the small metal heads quickly with your blastr or scatter gun, and avoid the leaders single shots of concentrated dark eco.while shooting and running from the smaller metal heads, keep turning towards the center of the nest to land some shots on the suspended boss. When you have depleted 1/3 of its health, it will momentarily lose some supports and limp. get ready for part two.
2)Part two is identical to part one except you don't get any time at the beginning and flying metal heads will come up from the hole in the center of the nest. Also, the metal head leader will give a double shot of concentrated dark eco, so be sure to jump or roll away twice when evading its shots. Once you diminish the 2nd third of the boss's health, it will jump down from its suspensions and part 3 will begin.
3)The metal head leader chases you around the nest, pounds the ground causing a circle of dark eco to come from it which you must jump over (like shockwaves from other games), and will shoot a triple shot of dark eco. If you do not have enough dark eco to turn into dark Jak, just keep shooting the boss and keep evading his attacks. Otherwise, mutate into dark Jak, transform into giant dark Jak, and administer a dark bomb. This will deplete more than half of the monster's remaining health, and is a good head start on finishing him off. After that keep evading and shooting until the beast collapses.
This may take a few times, so if you get low on blaster ammo or need to collect more dark eco for dark Jak, waste a couple lives collecting ammo and shooting all the small metal heads to collect dark eco. After you defeat the boss, sit back and enjoy the ending movie.