On Arthur specifically, 43 HP, 32 Atk, 29 Spd, 30 Def, and 24 Res as a 5* means his standout stats are HP, attack, and defense while speed and resistance are average. Attack and HP would be above-average and defense you could argue as either high or also above-average. One crappy thing, but not as crappy as with M!Robin having 4 stats that can drop down by 4 points is that -Def and -Spd drop him down 4 points. Going from 29 speed to 25 is rough and so is going from 30 defense to 26. On the flip-side and I'll talk about this later, +Res increases his resistance by 4 points.
My main Arthur is +Atk, -Res and he's mostly there to hard counter lance units. In the long run, 35/29 offensives if he were a 5* unit isn't that great when other units naturally have similar offensives or higher. Some of them are glass cannons; some of them aren't like Barst comes with a Brave Axe and 33/32 offensives and has 30 defense like Arthur.
+Spd is okay, but I feel like he's going to run into the same issue as him being +Atk. 32/32 offensives pales in comparison to Barst's 33/32 and if you go with -Res, then Arthur would have better resistance than Barst at the cost of having lower physical bulk with 3 less HP and 21 resistance drops him to being among the units who have meh resistance.
+Def is cool, but summer Xander exists with his 36 defense and similar stat spread except for having 8 less resistance. Granted, he's limited. There's also BH Ike, but he's a thing... And once again, you could run a +Def/-HP or -Res Barst who'd keep his stronger offensive spread for having much worse resistance. Alternatively, if don't care about resistance and you don't care about doubling, Bartre, Cherche, Frederick, and Michalis all have higher attack and defense by default. Also, if you keep him with Emerald Axe, 33 defense might not be necessary.
The one thing I remember being mentioned about Arthur on Serene's Forest and it was funny enough, a reason why he's better than Barst -- and Bartre and Raven being better than Barst -- is that Arthur has good resistance. By default Arthur leads Barst by 7 resistance for having 3 less HP and 3 less speed which does get evened out when Barst has his default Brave Axe. If you consider 20 resistance to be the actual average because it is the weaker defensive stat, Arthur having 24 resistance is pretty good. +Res giving him 28 resistance is even better since this makes him, Amelia, and summer Tiki, one of the few axe units who can have almost 30 resistance. Granted, they cap at 28 resistance or 27 in Amelia's case and Arthur and Tiki don't have access to movement type buffs like Narcian, but it's still pretty good. In the case of gem weapon and T-Adept users who have good mixed bulk, -HP works for since they're using whatever defensive stat to lower the damage they take. So, Arthur dropping down to 40 HP, but having 30 defense and 28 resistance could let him wall out blues.
The issues with running a +Res Arthur is that other than needing Distant Counter to counter blue mages at range -- not really necessary if you just want him to tank hits -- are his speed being average and that there are other units with higher resistance by default. For speed, the only lance units he can naturally double without Lancebreaker 3 or speed buffs are Gwendolyn (24) and Berkut and Lukas (22). Effie has Wary Fighter and Jagen having Fury 3 puts him at 27 speed. For blue mage, he can only double Reinhardt, but if Reinhardt is mysteriously +Spd and not using Dire Thunder or is even -Spd with Dire Thunder, but has received a Hone and Goad Cavalry buff, then Arthur cannot double him. Granted, he can possibly one-shot most of the blue mages and lance units with low defense, but he gets doubled by every blue with +34 speed. That will become a problem if they have specials and unlike Titania, Guard might not be a great idea if he needs a -breaker or Quick Riposte for damage output. Granted, that's if you're trying to have Arthur hard counter blues rather than having him wall out blues and/or hard counter lance or blue mages.
Resistance is self-explanatory; Arthur caps at 28 resistance while other units start at ~28 resistance. In that case, waiting for a new axe unit like Libra might be a better idea if you want someone who has high neutral resistance, but that doesn't mean Arthur can't be a good blue mage wall by being +Res.
Anyway, if you're having him wall out blues exclusively, choosing between Bonfire/Luna and Ignis is up to you. Bonfire/Luna activate sooner with Bonfire being consistent in damage while Luna scales better with higher merges and PvE units having higher defense. Ignis would be better if you're looking to explode a unit after building special charges, but that might not be ideal and Arthur might not have the stats to do so unlike Selena who uses her high bulk and speed to mitigate damage, avoid doubles, and double back against melee units to grind out battles; +Spd Selena with Fury, Renewal, and Ignis.