out of these three swords:
hammer, acheron*, btb
*acheron or obsidion edge (which one should i pick out of the two)
which combo should i pick
hammer + acheron
hammer + btb
acheron + btb
also which guns should i pick out of these
obsidian carbine, polaris, driver (storm, shadow or magma)
which of these should i pick if i had to use it for everything i do? vana, jk, arcade....
Knyack knyack knyack
Being the gunslinger I am, I'll have to say the nova driver will be your best bet for everything. It will get you deeper for the long run of FSC and it's decent compared to it's brother the umbra driver for Royal jelly palace. But since you said one it has to be nova for the pure damage it gives.
Other guns should eventually come to you as well later on, as having just one limits many combos. Hope my little tidbit of information helped, cheers.