As a German, I can slap -in on anything that shall be gendered. No exceptions (or at least I can't think of one at the moment, feel free to make me eat my words). The English version is -ess, but "deviless" is completely uncommon (although in the dictionary). "Devilette" is also used (as rarely), "she-devil" is almost always metaphoric. Methinks everyone (10x more Google hits) weasels out by using "demoness" (which is technically wrong from the mythological standpoint, but since you rarely meet one who gets annoyed by the wrong moniker...).
Which option is the best here? (And are there more person nouns refusing -ess? Besides regular -ette and -trix forms found in the above link, I mean)