I følge kunnskapsforlaget store ordbok

Context would be good - but if superior skill is meant, then dyktighet, ferdighet, possibly even 'overlegen dyktighet' might fit. Or lack thereof, as the case may be.

As mentioned before; it is hard to give an exact suggestion without more context, but in a litterary context (although it is more oldfashioned) prowess can also mean "tapperhet i kamp". (If you can substitute prowess with bravery in your text, then this would be a valid suggestion for translation.)

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Note added at 1 day3 hrs (2007-05-24 13:35:14 GMT)
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Depending on context, it can of course also mean ekspertise/ dyktighet as mentioned by the good people above, so I agree with both of them as well. :)

Ex: "det var en bragd å få arbeidet så fort ferdig"
Ex: "utføre en bragd, et storverk"
Ex: "en fremragende kunstnerisk prestasjon"