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>>280210818
>The second problem with the plan is that there is no in-universe reason that GS should know this was possible.
You are right that on his own, Goblin Slayer would likely not come across this knowledge. But he is familiar with Witch and she is happy to help him and give him knowledge for the right price. As for the deep sea pressure and water cutter ideas, I do agree that the main series doesn't address how the knowledge came to be, simply that it exists. Perhaps a curious wizard tried this in the past and made a mess of things and that's how the information was spread, but that's just a theory. There is also Year One, the prequel series, does address this and reframes the idea from high water pressure and cutting to a more simple "If you would drown at the bottom of the ocean, then bringing the bottom of the ocean to the surface could be an offensive weapon." We see his first use of a Gate scroll here, and from the aftermath we understand why Goblin Slayer was confident it would work on the Ogre. You can dismiss the Year One moments since they came in a prequel, but I figured I may at least mention them.
>This is just a scroll he bummed off of another adventurer.
One last minor thing, but he gets the scrolls from either finding them in dungeons or paying the Smith to collect any he comes across. He then gets Witch to assess them.

I do like this series a lot, so reading and participating in the discussions about it is fun.