Cannot be missied.
Although it is advised to fight and kill the boss in the opening sequence as you gain more of an idea of how the combat of the game works.
L2 is your aim button
R2 is your fire button
O is dash/evade.
These are all we need to know for the boss. He is very predictable and often uses the same moves in the same sequence. Keep dashin sideways to O and then unloading in a quick volley of fire then repeat. You can run around normally too, I found that was effective but uneccessary.
Once you've got passed him the game expects you to die anyway, so feel free to let yourself succumb to the void and end the almost-prologue.