Which one is grammatically correct?

I think its, My life is a nightmare because I would write the first two like : He was AS STRONG AS STEEL/NAILS. not sure how the godlike speed is wrong but I know that the life is (is correct) and your using a instead of an which is also correct (e.g: if you said my life is ___ animal (not a good word choice) in the blank it would be AN because animal begins with a vowel.