Is there a Norwegian word for "sexy"?
You mean aside from the terribly original "sexy"? ;)
Other options are "heit" or "deilig", as well as a bevy of slang words which are usually quite socio-/dialect dependent.
If looks could kill.
so the word for the adjective is the same as the adverb?
Here it is, yes.
What about "she is pretty hot"? I think it gets the meaning very well, and it is possible to make that combination of words in the exercise anyways :-)
That's a weaker expression.