In order to defeat Secto, you must go through two of his spider pets. Shoot them with Sting Bees and Bombats. If they get too close, hit them with Riot Slugs until they disappear and run away. If they catch you in their whirlwind attack, just aim and shoot until they run away. After you defeat them, you must fight Secto. When fighting him, hide behind the big rocks until he shoots you with his giant laser. After the rock breaks, shoot one of the two power generators at the bottom of him. They have blinking lights on them. Shoot them until one of the blinking lights gets broken off, then run and find another rock to hide behind. You must do this about ten times. If you need more ammunition, there is plenty around. Just wait until after he breaks your rock, get some, then hide again. After you destroy all of the blinking lights on his power generators, he will die.