Why Arthur...? Please don't. She's a pretty awful Soul unless you can use her instant-burst gimmick, and I doubt that you will ever be able to do that.Why...? Please don't. HP Soul weapons may have better effects, but outside of Dartagnan (or Shingen for a very, very specific purpose), please don't use them.
If you absolutely must use the Arthur for some reason, get the offensive weapon. Yeah, -1 CD for the nuke is nowhere near as good, but 30% Elemental is SO much better than 30% HP that it will not pose a problem. Better yet, just get and use Herc Axe or Shingen Lance like everyone else.No you don't!No you can't! They don't even fulfill the same role!!!
But if you don't want Uriel, fine. Just know that you're making a mistake, but fine.
What should you get then? It sounds like you want to commit to Fire, so you'll be using it off-element? In that case, I wouldn't recommend Dakki. She is pretty damn good against Wind, but not quite MT worthy for a team that will fight against everything. Amaterasu completes Fire with her amazing debuffs, but the debuff-doomsayers don't like it. And Amaterasu isn't very impressive without her debuffs. Ares and Acala are NOT Miracle Ticket worthy; they're decent, but that's all.
That leaves Mars. Debuff-doomsayers would disagree of course, but at least Heph+Mars+Sniper Shot would allow you to get reach -50% Def. And SS shouldn't even miss much after Mars' debuff.