Boss: Centurion (huge centipede) | Umbrella Training Facility Walkthrough Resident Evil Zero HD Guide

You need to fight with Centurion (a giant centipede) as Billy, because Rebecca has been kidnapped by the monster. You do not need to hurry too much, or risk your life at all costs, just to win the battle. To win, use the grenade launcher or, if you do not have it, the shotgun. Do not attack the monster when he is moving, because you may miss him and, apart from that, the monster's upper armor is durable. Wait for moments, at which Centurion stops and raises the front part of his body (just like in the above screenshot). This is when you need to shoot towards the exposed white underbelly.

Centurion is a weak boss in you should be able to kill him quickly

Right after you fire your weapon, move away, because the boss starts moving around again. Try to avoid getting in the monster's way, as well as being knocked by it, as a result of staying too close. Wait for the moment, at which the boss stops and the situation from before repeats. Depending on the weapon that you are using, and difficulty level that you have selected, you can win quickly or this will take some more time. After you kill the boss, remember to check out health level of both of the characters and, if necessary, heal them(just to remind you, in the next room with the ravens and the fountain, there are plenty healing plants).