I think it's a physical attack strenghtened with magic. Bell called it dual charge if I remember right? Normally he can charge only one thing at the time, but he managed to get it work after Hestia knife absorbed flames from firebolt and then he charged it. He is swinging a knife so it's physical, but firebolt is still there so is magic. So I would say it's both.

It was said that argo-vesta was neither of them, neither physical nor magical. And it is not that Juggernaut has a weakness, his defense in itself is his weakness. when Bell uses argo-vesta he throws a blaze of fire, however, the attack is not magical in nature, I think his best definition would be an unknown attribute that was created from a physical attack embedded in magic, however, he didn’t can be classified as neither. The author has not yet explained much about him. And I'm also interested in what would happen if Bell used argonaut on a magic sword, could he be considered an argo-vesta too?

And that is the problem. The only types of attacks that we have seen so far in the series are either physical attacks, magical attacks, or attacks imbued with spirit magic like Ais uses.

The author has not clarified what this attack actually is which is lazy if you ask me.

Also, the Juggernaut does have weak points as Ryu's familia members were able to damage it enough that it retreated. Read volume 14 synopsis with the flashback from Ryu. It suggested that they were able to get past its high physical defense as well as it magical defense to actually damage it.

I believe that during the fight with the Juggernaut, Bell actually attacked an unprotected area of its body with a point-blank fire bolt and it damaged it from the inside. This allowed him to pry off some of its armor so Ryu's magic could work on it.

It was not so. In Ryuu's flashback, Alise had damaged her shell in her suicide attack. It was said that in exchange for his high mobility and the fact that he had no magic stone, he had a relatively weak defense. apart from his shell that reflected magical attacks he did not have much defense. Bell destroyed his shell in his fight, next to his arm, if I'm not mistaken, so later on Ryuu managed to use a magic attack on him. I personally think that the classification of Juggernaut would be like this: strength and speed lvl 6, defense and resistance lvl 4, which results in its classification as lvl 5. And in my opinion, I think that the author did not explain much of the attack because there was already a lot of content in the books, and there was no need to go much deeper in the technique so early in the story. he must not have said much to leave loopholes for future changes, maybe another technique or new use of the double charge, whatever.

Now I think I remember the deal with the Juggernaut. In return for its strength, speed, and magic reflection ability. Its physical defense was weak probably because getting within melee range of one would be suicidal.