Then here's my plain opinion: the Latin idiom will change depending on what you mean or want to say. I'm not sure what you mean, exactly, by Never give up until the end. The end of what? Life? Something? Anything? The Romans were most likely to say, "[I beg you] Do not cease from your present task/enterprise/work." Above, I've made this to be "Never cease until you've completed [this matter referred to above.]" Here, there are still many ways to fiddle with the translation a bit--again, depending on what one thinks he means/you mean.
In any case, the translation "Numquam cede, usque ad finem" is wrong word-by-word and as a whole. Don't use it. Even "ne desinas" is better: Do not stop. (ancient proverb)