Don't need one. How did olaf one eye and Ysgramor get into sovngarde? They didn't die in battle. Or Jurgen windcaller. Heck, Jurgen dies a greybeard, and they aren't violent unless they need to be, and seeing how they dont describe his death, it seems likely he didnt die in battle. That may not even be a requirement more than just living a life of battle as well as dying in battle or valiantly.
Yea that's right. Silence.