I have good enough matchup exp vs Arthur now that I can really work with it. Heres what you need to know:
Fireball stuffs 3 daggers and dies, 1 lance and dies, 1 boomerang and continues, a few bracelet balls and dies, fucks up the bottle, and will nullify most of the weaker beam assists (my friend uses Chris.) Make use of the full-screen fireball confirms that are floating around in this part of the forums. Having two fireballs out mows through his wall, giving you some space.
Try running a beam-super assist and punish his keepaway assist with THCs. Because Arthur has to chuck each fireball one at a time, and because he doesn’t have an unfly for safety like Morrigan, you can force him to XF, burn bar on armor, or take the hit/a possible unblockable setup depending on your team. This is worth stocking meter for.
Lots of Arthurs use missiles for the lockdown. GET IN BEFORE OR DURING THE MISSILE CALL. A good plan is probably to Bon Voyage behind a beam assist or, I don’t know, Dormammu 3D0C/2D1C. The goal is to lock Arthur down -before- he gets momentum, because he can very easily follow you up into superjump range and harass you with axe, spear and j.H.
Since he’s small, lots of your shittier gimmicks will fly by him. Test all of your setups on small people, and make proper adjustments. Doom and Dorm players don’t give a damn about size, but Ammy and Sentinel players probably should check their shit, because Arthur ducks a lot of things.
When you’re doing the standard air-tech unblockable setup with Amaterasu, they might tech forward and j.S. Every character can do this, and it knocks Ammy out, but Arthur’s j.S brings him down, so he’ll immediately be able to punish you for trying to unblockable. Let go of the swoop early, or cancel it into Lumenize, to make it safe. If he doesn’t have three meters, it might also be a good idea to use your ground unblockable setup. If you don’t have one, watch a Zak Bennett match video.
Best of luck!