Boss 4: Proto Tyrant - first encounter | Boss battles Resident Evil Zero HD Guide

The first of two encounters with the mutated creature takes place during the final phase of exploring the factory. After Rebecca uses the large elevator she will meet with Enrico and take a key to the new, smaller lift. Attempt at using that elevator will automatically start the battle.

Attacks of the Proto Tyrant can knock Rebecca to the ground and seriously wound her

You must defeat Proto Tyrant. The bad news is that the duel will take place in the smaller eastern area (M5,9) which was separated from the large elevator with a wall. Just after the battle starts try to quickly get away from the boss as he is performing strong attacks with his upper limbs. These attacks can knock down Rebecca and deal serious damage to her. Try to keep distance from the boss during the whole battle. If Rebecca gets hurt, quickly open the inventory and use medicaments to heal her.

It is best to use Magnum for defeating the boss

The best weapon for defeating Proto Tyrant is the Magnum pistol which you have recently found in the cable railway's cart. Depending on the boss location you can shoot few times or just once before you must change your current position. Remember to avoid wasting ammunition for the Magnum. On the other hand, don't use too much time on aiming as the boss is very agile and he can quickly get close to Rebecca.