As a swordsman, who's only non-sword weapons are:
Blitz Needle for Trojans and Vanaduke
Valiance for switch shooting (more convenient than Blitz which stops you from moving)
Nitronome for pushing enemies away and making small safespots by spamming it one place because of the knockback
Shivermist Buster for FSC and Vanaduke
I ask, would there be any real use to have Callahan and Sentenza from the perspective of someone that uses swords for almost everything?
Also:
If AP becomes pure Elemental damage, would that mean it would become a resisted gun against Vanaduke?
If so, what gun would be best to solo Vanaduke with?
I can't think of one. Unless there's a specific enemy (i.e. Alphas) that bugs you at close range, then just stick with the swords.
After damage change on AP/Sentenza, AP at pure Elemental will be resisted. At that point, there will be exactly three piercing guns: Blitz Needle, Plague Needle, Callahan. Not sure about the viability of Callahan for Vana soloing, but assuming it's not viable, then the best gun may still be AP even with the resisted damage. Or possibly the Valiance, though also resisted.