kinda impress me how you found the way to beat hag by reading through this thread, but not on the "use particular item in a specific spot" bit cause lot of people also ask that so i'm pretty sure you would've stumble upon it....
anyway, remember the Scivia powder you were told to craft oh so dearly? yeah that's the item you need to use. but where? top left on the map there's a location with few pillar, go there and use the powder.
be aware that this will instantly trigger a boss fight, arguably harder than the final boss itself as final boss is all about attrition battle but this one have you fight difficult secret boss, prepare well.
as for the slug, nothing much, apparently just paying homage to the 1st game. personally i would like to believe there's some kind of lore or explanation about this slug
