This is from an Faq on gamefaqs.com Hope it helps

Objective: Defeat the Zombiebot King and his minions in order to save Mozer
from the giant bot chipper.

Boss time. First, you want to get Zombiebot King to break the grates open
that cover the holes in the ground. Collect the pickups out of these and the
secret chip across in the hole that's across the arena. Now set your focus on
the boss himself.

Collect as many RL rounds as you can (learn their locations). Shoot at this
boss until he stops and leans forward. At this point equip your slingshot and
shoot a coring charge into mouth. You'll know you made a direct hit when he
groans in pain and goes on the attack again. Do this three (3) times and one
of his Berserkers will come out. Destroy it and collect its energy. Attack the
Zombiebot King once more and hitting it in the mouth and two more Berserkers
will appear. Take care of them and finish the Zombiebot King off with a fifth
and final attack to the mouth.

TIP: If you need energy, shoot the artifact hanging into the area left of your
starting position. Full health energy will fly out.

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