[quote=@thewizardguy]A quick clarification.

Daemon are not incorporeal. They are purely physical entities. Their body is solidified warp-stuff, but it fulfills the same general purpose as the bodies of normal mortal creatures. The only difference is that Daemon have no internal organs, and therefore can only be killed by inflicting massive trauma on their physical forms (and certain magics). More powerful Daemon can also alter their form, regenerating from damage far more quickly than most mortal creatures. When enough trauma has been inflicted the Warp stuff no longer holds together, and the Daemon is destroyed, returning to the Warp. The ability to harm incorporeal or ghostly entities does not increase a weapon's ability to inflict harm to a Daemon, with the exception of some specific incorporeal warp entities.

The reason for Magnus' ability to reduce damage impacting him is the fact that his body is magically 'custom made'. It's unbelievably sturdy, far more so than almost any normal physical material. In addition, something akin to a psionic forcefield covers his body. This field vastly decreases damage inflicted to his form, granting him immense resistance.

If you want a weapon that will really cut him up, just use the spear that slices through all magic. This would ignore not only his magical forcefield, but also his armour and his physical form's durability, as all of those are magical.
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All that basically applies to normal Servants (minus the vulnerability to Gae Dearg), which is why they have an END stat. It's just that they share the same sort of minimal threshold for dealing damage. Point being, none of his weapons are less than the weapon that has already hurt Magnus.

Besides, there's a [url=http://typemoon.wikia.com/wiki/Harpe]far better choice[/url] if I wanted to be a dick about this and not just get it to the point that Magnus might realise that this isn't a fight to stay and fight out.