To defeat the large monster in the wooden area, drop to the right immediately after you go into the pit. Let him charge at you until he is in the corner. Stun him with the pistol, then run back to where you started. There should be a flammable barrel nearby. Shoot it and try to catch him on fire. This deals significant damage, and you should only need several more pistol shots and a couple shotgun blasts to finish him off.