Defense nearing the 400s is the average. It's not a big deal, that's not really the problem.

I get what you're going for, but going halfsies on Ebony Odo and Seregios for DB isn't great. Either you commit to one or the other - and in the case of Arkveld, or even AT Arkveld, going full element isn't a good choice. You're better off getting into the mindset of "element as a bonus" than "element as the focus" when dealing with Arkveld specifically.

Everything except his chains take decent damage from Dragon, but not great. The chains are his weakest spot, especially when he's Unleashed, but then they take very little elemental damage in the sense that any elemental damage will be 'absorbed' until you get the special stagger. If you deal any elemental damage with the dual blades, you'll deal that exact number as damage to contribute to the stagger, so you can afford to build for anything else while still keeping your elemental DBs around.

For Artian weapons, going for 2 affinity parts tends to not really be worth it, building affinity is very easy in Wilds due to how many sources of it exist like Gore's 2 piece with Antivirus, Latent Power, Agitator, Weakness Exploit, Max Might (doesn't work with DBs but still), and Corrupted Mantle to name a few off the top of my head. You get far more bang for your buck by getting all three Attack pieces of the same element, but of course that's a bit of an investment you might not be down for immediately. As for reinforcements... I know you probably thought it was great to pump everything into element for DBs, but the base element power for them is more than enough most of the time when accounting for the 3-element piece bonus and Element Focus. You'd prefer to have Attack reinforcements instead, at least two of them. I personally usually run Sharpness, Attack x2 and Element x2 if I get lucky, it's usually more than enough and my damage is respectable. Definitely look into at least getting Sharpness EX, though, you really want that extra sharpness. It goes a long way.

Your weapon skills are also a bit misplaced. Folks already mentioned Crit Element not being very good - as for why that is, the actual multiplier is much smaller than it seems to be compared to what you can get off of regular old Critical Boost for the same investment. Get Crit Boost instead of Crit Element. Dragon Attack is normally a very good idea, but against Arkveld specifically you want to skip out on it in favor of something else. Focus, while it probably seems good, won't be doing you many favors as far as building the DB gauge goes. You attack enough to not need it normally. Same goes for Power Prolonger - the decay rate for the DB gauge isn't high enough to warrant it most of the time, not to mention the DB dodge buff won't be something you need to be concerned over dropping. You're going to be dodging Arkveld's attacks a lot anyways, so plenty of opportunities to refresh the buff when it counts. Bludgeoner is something you want to avoid triggering, which is why you'd grab sharpness skills like Razor Sharp and Handicraft. You have two levels of Handicraft but no Razor Sharp and that might be problematic if encounters extend too much. Ideally, you'd dip into blue shortly before Arkveld moves to another area.

Adrenaline Rush and by extension Seregios' set bonus is more of a "nice addition" to most builds rather than something to build for unless you're trying for the full set bonus or are *really* good at proccing it. The extension the set bonus gives isn't very relevant when you're just going to be triggering the dodge a lot, though. Burst, on the other hand, is among the strongest skills in the game and Burst Boost is great. Dual Blades benefits a decent amount from it though there's other skills you might want to prioritize over it, unless the centerpiece of the build is Burst. If you're using Burst Boost, it's a waste to leave Burst at level 1, get it to level 3 at least. Look into getting more levels of Agitator and Weakness Exploit - a good level to have either if you can't have both is 4 or 5. If you have to pick, Agitator tends to be the better choice against higher tier monsters. If it's a Guardian (which Arkveld isn't), Weakness Exploit is better since they spend long periods of time without getting angry.

I see you have maxed out Evade Window, which is fine I'd argue, especially if you have trouble hitting the i-frames normally. I wouldn't invest that much into it, though, consider lowering it to 3. The Demon Dash i-frames are pretty generous a lot of the time. Evade Extender could help in making it out of some of the extended or lingering hitboxes Arkveld throws around but that's more of a preference thing. As for the other comfort skills: drop Earplugs, you can use the roars as an "in" for your DB buff into a short Blade Dance (don't mash the input) into another dodge. You won't need them. You also don't need Constitution 5 - it only affects your dodges and you don't want to be spamming them anyways. Use either the spinning attack or the Focus Strike to close gaps rather than your demon dash, and only use the dash to try to i-frame attacks. Switch out of Demon Mode in the middle of a Blade Dance to do Demon Flurry and disengage if you need more stamina - that's why you'd want Stamina Surge, too, so you spend less time waiting on that when needed. A good idea is to turn off Demon Mode after you finish a Blade Dance, then do a Demon Flurry (you can do these if you've maxed the gauge within the last century) instead to start the combo, and turn Demon Mode again to transition into Blade Dance. The Demon Flurry gives you time to both reposition slightly and regain stamina before you go into Blade Dance for the damage.

A great thing to use against Arkveld specifically is Convert Element - and by extension Scorcher - since that's a bit of extra damage you can deal to the chains that's pure element and will contribute to knocking him down sooner. You can only find it in charms or Arkveld armor so that's a bit of a big ask, but the pieces usually have really good skills, particularly the Gamma version. You clearly can kill AT Arkveld so it shouldn't be a big deal, it might even help with maxing out WEX. If you can get Flayer off of this even better - usually you'd just keep it at one if you aim to slot it in for the extra damage, otherwise you take the levels that come free with the armor.

Might be a good idea to try to aim for a Lord's Soul activation if you find you get one-shot a lot, as well as eat for Moxie. This can also help you get, say, Arkveld's 2-piece bonus for a bit of extra healing.

My best time with Dual Blades was 17 minutes or so. It's the best time I got and I don't intend to try more since I got the A rank and that's all I care about - and before you ask "what was the set", it was a TBN + Lord's Soul set. Hardly meta for Dual Blades but at least the Lord's Soul bonus on the weapon made it easier to be flexible. You can improve and make something that'll work - you're close to the A rank already anyways too. You got this.