How to beat the boss...spoiler alert !

Just aim to the red spots near his eyes

Keep shooting at these spots until the eyes open, then hit quickly both eyes with your laser beam.

Bonus and spoiler No 2: get a new weapon by cutting the tail of the boss

Before finishing him, try to cut his trail, you will get a new weapon

Just try to stay behind him as long as you can, hitting his tail with your melee weapon, after a few hits the trail will fall and you will get a new bonus weapon, then finish the boss.