Guise is a noun in modern English, not a verb or an adjective. You could say He went out in the guise of a doctor, meaning "posing as a doctor". Please don't use that phrase though. It's not very common and would probably sound odd.
We can say "he was disguised as a policeman" but if we want to use the word "guise" then we will say "he was in the guise of a policeman."
You asked me not to use the phrase but which one?