In addition to kill/loss ratios, I think it helps if you are outnumbered. That's where I have won most of mine. If you have the larger army you will almost never win a legendary victory. If the armies are roughly even, it seems like you have to barely lose anyone while killing most of the enemy; I've had battles where I have lost around 100 and killed over 1,000 and not been awarded it.

Legendary is a lot harder to get than in the older games, which is a good thing. It felt like I used to win legendary status in every second battle in RTW!