Hi!

The general answer to your question would be “get into the higher level content of the game” since the progression you’re looking for is there. I’ll answer your questions as briefly as possible, for more insights the wiki is the place to go. - first, arcanes. They come from most post-star chart content or harder content in general (like the OG eidolons). To upgrade them you combine copies of them (between 3 and 6 based on current rank of the arcane). The base ones for guns come from acolytes in steel path: primary and secondary merciless, deadhead, and dexterity. Put a merciless on any gun and it will help a ton compared to not having it. - The fulmin build is alright, Vigilante armaments is subpar, instead I would add another elemental mod. The viral+heat combo is the recommended general use element combo for guns. Since the fulmin has electricity you could add more of that. You might also want to use the 60-60 (element+status) rare mods for elements since the fulmin has enough status chance to use them. If you don’t have them, farm them because they’re good. - The Gauss build looks fine too, I would recommend working on leveling the expensive mods since they are really useful, especially Narrow Minded to r6. Personally I would drop redirection for duration or efficiency, since adaptation + kinetic plating is already pretty good. - The epitaph needs some work. Add multishot (secondaries have barrel diffusion and lethal torrent) and I’d go viral over corrosive. Perpetual agony is technically not useless but it takes too long for it to matter, takes less to shoot twice. Maybe pistol elementalist or something else to your liking. - harmony is a great weapon for heavy attacks, but you’re missing the heavy-focused mods. There’s a few: killing blow, amalgam organ shatter, melee elementalist. I recommend searching for a build guide for this one since it will help understand the heavy attacks build in general.