Update: Copen in GV2 can. I had Mark play through the games and we both kinda skipped the dialogue because yeah. I forgot a detail.
Normally, with Copen's EX Weapons. He replicates em, as seen in LAiX
"Using data from the fight, we were able to imitate that Falcon's Septima to create a new weapon: an EX Weapon"
But in Gunvolt 2 (this is 8-4 Translations so take this with a grain of salt, I can't find the better translations anywhere)
"The EX Weapon System lets me install Shards into Lola. Then she can use them to recreate Septima"
For context, the Shards refer to Zonda's Phantasmal Mirror, her Septima. They contain the Muse Septima, as well as the original holder's Septima. If Lola uses those to create Septima, then they are likely very legit and are the actual thing, instead of the Pseudoseptima he normally creates in GV1 and GV2.
This fact may be subject to change though as I do more research, just in case... We should likely put this in Copen's page.
Still... Caladbolg gg... Copen's Septima "resistance" does sort of work, but it's more of that it doesn't let Septima screw with his technology. In the scan that Septima Resistance links to (which is legit btw, comes from Dangeki Profiles provided by Inti themselves), it specifically is talking about Milas, the water Septima user trying to short circuit Copen's tech, which doesn't work. So it's not that Copen has resistance, it's that his technology has that resistance, assuming further would likely be NLF. Besides, these resistances are built up by actual experiences in battles, Copen has never fought a space warper before (if you wanna count analyzing Jota... Fine? But he only really got the light portion out of it instead of the whole ripping space time. Merak never used his space warping offensively either, so that's a no go). Even after all of that, even a 100 years later in LAiX, Copen wasn't able to get a good defense against electricity Septima for his tech, so it's likely that this resistance is just for general things that can screw with tech and not for every little thing, especially something as strong as Space manip, warping a part of reality itself.
We gotta revise that part of the profile... Yeah.
Of course, the possibility of dodging it still exists. Caladbolg II hit through Medea's teleportation, but that was because like, Medea's teleport was space displacement if I recall. Not sure if that's the case, but if it is, Copen could potentially Bullit Dash to dodge, which gives quite a massive speed boost. But then there's Eye of the Mind and the massive gap of experience. Copen is 14 in Gunvolt 2. Archer has been fighting for longer than Copen has even lived. I'm not gonna say it's a stomp where Archer fires one arrow and it's GG since I don't think he'd start with that (Caladbolg takes a decent bit of sauce if I recall), along with the charge time. So Copen has a chance here-
So I guess here I go with Copen's points:
First off, Prevasion. Well, that's pretty much the most significant thing he has. Pretty much none of Archer's attacks work against it if I recall all of his weapons aside from Caladbolg, letting Copen cross the distance unscathed until he does so. With his speed from Bullit Dashing, Copen is likely gonna at least tag Archer some of the time, though it'd be hard with Eye of the Mind. He'll need attacks that cover a larger area for that... That is the case, if he can even get close enough for CQC in the first place. But, if he does get a tag in- he has a one shot of his own. Hailstorm Blade is a weapon that generates "below absolute zero" ice (official websites states that, as well as Dangeki), causing those who are hit by it to lose pliability and shatter on contact. But considering that kiting... Exists since Archer would see Copen approaching and back off. Eventually, Archer would see his arrows don't work in this cat and mouse game, and use Caladbolg eventually.
Flashfield would deflect normally arrows, but all Archer would have to do if either use a sword arrow or just charge up his arrows more. We've seen swords and crystals ignore the barrier, it's safe to say that Archer's stronger attacks could as well.
Healing is very effective, but it basically is just delaying the evitable, nothing will really change since Caladbolg is just a one shot anyways
Copen won't get many, if at all any chances to get his OHKO off. While Archer has access to his own at all times. But still, Prevasion makes all the difference here so Copen really does have a chance if he closes the gap. Copen still does have an answer to this with Vantage Raid, which binds victims in place when the projectile hits. But considering it's a linear angle of fire, Archer won't have a problem dodging it or destroying the projectile, after all, he's Archer, a Servant class skilled at ranged combat, while Copen is defined to have a closer range style given his style involves slamming his gun into them, just that he has tools that work at long range less effectively.
One last thing up Copen's sleeve, Bullit Dashing, his main tool is a very great tool to have for gap closing. However, it's also pretty linear, which means Eye of the Mind comes into play. Copen CAN get close enough, but it costs a lot of Bullits to do so, he's gonna have to reload often (which stops his movement a bit, if he's in the air, he'll fast fall stomp into he ground... I gotta add that to the profile too... Ugh), giving Archer time to run away some more. Copen's still going to have a hard time rushing him, the point still stands that Archer's OHKO can be attempted any time, and Copen's can't. Sure he has the versatility to help him get to closer ranges via something like Vantage Raid, but again, not very relevant against Archer.
And if he DOES get in, Rho Aias would stop a Bullit Dash in its tracks, all of this while Archer is probably chanting UBW recognizing that Copen can beat him if he gets close, which at that point... He'll have a lot of Caladbolg's to work with.
TL;DR: Both can get one shot. Copen's OHKO is close range, Archer's is long range. Copen can't get in easily, even with Bullit Dashing (cuz he needs to reload). Archer can't land a hit easily. But Archer has more chances to actually land his, and Copen doesn't. Archer has experience advantage, and Copen is on a time limit because UBW.
Archer high-diffing imo. It's a dangerous opponent he's up against tho.