The WSOY Finnish-English dictionary offers as translations of "melko": fairly, rather and quite. These seem reasonable alternatives.
"whoa" and "woah" are both given as acceptable spellings for translating "huh", but the answer with "whoa" is marked incorrect. (IIRC "whoa" is the more common spelling in English so if anything it would be the other way around)
This may have been fixed since you wrote your comment, since the suggested spelling I see now is whoa (which is indeed the one listed by Merriam-Webster).
Please Duolingo, be more tolerant as for the spelling of English interjections. It is a course