Sh getting all items

I’m replaying silent hill for the second time now,and i wanted to get all items.

It says that to get the green beam alien gun,i need to get max rating in the game?
It also says:That you have to use the channeling stone to get the weapon.

I want to get the katana next.I got the channeling stone chainsaw and gasoline last time.

chainsaw has limited uses right? gasoline runs out fast? How do i know how long the weapon lasts with gasoline? is the rock drill better?
I heard that people usually don’t use the chainsaw right,and that its the most powerful melee weapon in the game.I saw a guy use it against nurses and doctors and totally rape them.The guy said,not to use it against dogs and those flying gargoyle things.