Rock crabs drop 10 charms a time so I'm going to have to say killing monsters. It might be another monster but it's a close bet it's around what I said.
Go grab a zamorak spear / sara sword and smack some waterfiends, that drop Crimson charms 90% of the time. If you can't afford those, get the best weapon you can find that has a crush option.
If those are tough, just range iron/steel dragons, they drop 3 charms a pop and have visage chance.
Rock crabs drop 10 charms a time so I'm going to have to say killing monsters. It might be another monster but it's a close bet it's around what I said.