I was wondering if 닝겐 is pronounced 닌껜, same for 멋대로 as 머때로 and so on? Is the "g" in ㅇ pronounced differently than the "g" in ㄱ?
'ㅇ' in Jongseong(종성) is pronounced as ng[ŋ], as one character. So it shouldn't be separated to 'n' and 'g', thereby '닝겐' should be pronounced as ㄴ(n)ㅣ(i) ㅇ(ŋ) - ㄱ(g) ㅔ(e) ㄴ(n).
This word comes from Japanese word にんげん (meaning 'human' [인간]), which is pronounced NEEN-gen. This is how I would basically pronounce 닝겐. Because 닝겐 is a transliteration of Japanese (which has no -ng sound), I would suppress any emphasis on the 이응 받침 (ㅇ on the bottom of the first syllable).
As far as how "닝겐" is pronounced, unless you are intentionally want to speak in different way, I'd say native Korean language tongue would have no problem with just saying "닝겐" as it is.
닝겐 : ning - gen / gaen 닝: ning, 겐 : gen / gaen 니: ni ,o: ng