You can create advanced guns, though nothing that can't be transported on a non-brute person. If you have a Sniper Rifle you made enough time to charge, say about 10 minutes, you could probably damage an Endbringer's core. Unfortunately, that means you'd have to charge your Sniper Rifle before the Endbringer battle and you'd only have one shot. You'd also have to know where the core was, and the Endbringer might be able to dodge it even if you aimed correctly.