The only advantage the sword can give is Feraldo's skills when he train you with the sword, still you need to grind up so you can truly fill them in. Arihama will be easier to defeat if you take the side quests. Spoiler:
Akonian Ritual - helps you uncover your lore (meaning there is no second round to fighting Arihama)
More Recruits - Your entire fighting capabilities of your troops will be boosted e.g: temporary defence from Skru'Vaks when he is behind the frontlines, Minor additional damages from other gun equipped troops (Rylan, Riordan, Chip, Phoenix). Frontlines are well frontlines (helpful in dealing damage and as well as receiving damage).
The Sythyrian Armor (less evade, more def).
I prefer the ranged weapon unless there is a reset respec stat in early game, plus the sword is too late and will took more time to strengthen melee weapon stat.
and also not to forget the lust stat which is the early strategy of the game, and most powerful stat.(In battling Arihama, the lust stat is like a stun/immobilize effect on him) while your crew damages him without the consequence of getting countered in return (but still hot af).