Here's one instance where I think a little creative license must be taken to preserve the spirit, not the letter, of the original Japanese. While the two Kanji that make up "zantetsu" do mean "cut" and "iron" on their own, they collectively mean "iron-cleaving." So yes, "Zantetsuken" does mean "blade which cuts through iron." Certainly clunkier in English than Japanese, but a little thought put into it can convey the same kind of meaning with a little pizzazz. For instance, the aforementioned "Holy Dragon Sword" is indeed a mouthful... the first thing that pops into my head is "Sacred Wyrmblade" (wyrm, of course, being a common literary term in reference to dragons).
How about, instead of "Zantetsuken," you use something like "Steelrender"? Most swords of notoriety tend to have their own names, a la Narsil, Hrunting, Excalibur, etc.