The first boss of the game is an easy one, but it gives you a taste of what the
bosses to come will be like. None of them will be just a straightforward,
shoot-em-till-they-die, type of boss. Each has a special method to defeating.
The boss has a couple of attacks at first. A blue "breath" attack that it'll
spray out of it's mouth and an energy beam. Jump over these attacks to avoid
them, locking on and strafing is a good method to help avoid. You can't deal
any damage to it's health meter until you get rid of the shield protecting it's
weak point. There are two noticable red orbs on it's shoulders, fire at these
both until they are destroyed, then you enter the next part of the battle.
He will occasionally fire purple orbs at you, shoot the purple orbs before they
hit you, and they will rebound right back at him. Once you've hit four orbs,
the shield protecting him will be destroyed, exposing his weak point. Fire as
many shots as you can, throwing the occasional missile or charge beam. If you
don't kill it now, just repeat the process until it's dead.
Later on in the game you will face two Berserker Knights (thanks Shade Tail for
identifying that) these Knights do not have shoulder orbs to worry about. You
can use your X-Ray Visor and Nova Beam to locate a weak point inside the second
Berserker Knight and you will be able to kill him in one hit.