How do I kill Many-as-One? I found a bottle of murk but I can't figure out how it would cause any harm to many.........

Well as for using the murky water you just put it in one of those quick item slots and use it from the game screen through the circle interface.
About Many-As-One... well just keep killing all the rats and hacking&slashing at Many too... eventually it'll surrender. It takes a while though.

So I found out..........thanks!