Once you climb the castle wall you will see Larry's truck. Load up on every single secondary item and get health and ammo to max. Go forward and you will encounter a monsterfest where you have to fight every single monster you have encountered so far in the game in waves. Go ahead and use only your melee weapon through the entire thing until you get to Goul Gang. Once you get to them take your Mini gun and let them have it. While doing this do not buy any ammo from those ammo machines over by the pillars. You can buy health though. Use your items where you need to keep you from dieing. You will get a checkpoint before the witches so don't worry about dieing and having to start all over again.