Zauriel is sent to stop doom once and for all, Doom is in his castle performing an old demonic ritual when Zauriel interrupts him by breaking through the wall.
Zauriel kind of vacillated in power a lot. In one 'true form', he basically resurrected the earth from destruction, but in a lot of his other showings he got taken out pretty easily. I'd say Doom probably takes it under most circumstances, especially lately where they've been playing up his level of power.
That said, Zauriel HAS defeated someone who had the Dr. Fate helmet/power...
The sword is powerful, but it has it's limits. It couldn't interrupt the energies being called down by the alien using the Helmet of Nabu, for instance. I think Zauriel could outmagic Doom, but I don't think it would be easy, and I think Doom's tech would probably push him over for the win.