It is incorrect.
Firstly, er is rather an adjective that means "alone", it needs to be min and (although adjectives should follow the nouns which they modify) cardinal numbers precede the nouns, so its position will remain as it is. However, "one day" feels rather idiomatic and may not really convey (when used in Elvish languages) the "at an indeterminable time in the future" sense with which we use it in English. In other words, "one day" could just be "one day", as opposed to "two days, three days" and so on. If you are fine with that potential mistake, you can use it. Otherwise, I think that mi lû "in time" may be safer.
As for cin, it is the accusative (object) form of the second person familiar/impolite pronoun (so with proper mutations those last two words would rather read "(s/he) will kill you"), it needs to go and the subject suffix -(o)g should be used instead. And I am not sure how you ended up with that ï in "fïratha" but it needs to be a regular i. Thus:
[Min aur / Mi lû] firathog.
"[One day / In time] you will die."