Before you got to bed at the Motel Room 6 you should have the Grenade Pistol or the M4A1, also have Energy Shot and Healing. The best place to be when fighting The Giant Beast is the Corner of the Room 6 and the Storage Unit he can't hit you there. Heal whenever needed and keep shooting have plenty of ammo though. If you kill the Beast you get a protein capsule (+5 health) and later from Mr. Douglas you get a full automatic Handgun w/ bullet capacity of 100!.