I could have sworn that the dictionary had a similar phrase, but upon opening the lexicon, I could find no such saying. As such, here is how I would translate your sentence:
Ke yaimpar o'r gar beskar'gam, ra draar yaimpar. (Literally, "Return in your armor, or never return.")
I kept "yaimpar" in the infinitive because, according to the grammar guide, verb+infinitive is used formally, and I figured a saying like this would be more formal than colloquial. Not sure if you would add the "ke" before the "draar" at the end, however. I assumed that saying "never return" was command enough in itself, but you may wish to add it to emphasize the command.