*After recent updates your sub city mayors will still die even if some of the troops survive... so there's not much you can do about prevent their deaths.
So a problem many players face is that their sub city mayor's die even after a win. This happens when your sub city troops are wiped out in battle, their leading general dies by default.
Luckily there are a couple ways to prevent this from happening.
FIRST OFF: Rebirth
This is a must have skill on all of your Sub Mayors in my opinion. This 5% is by itself generally enough to keep your Mayors alive.
Second: War Tactics
Cultivating your sub Mayor's Leadership and Politics stats to 900 unlocks an additional 10% death-to-survival rate buff!
These three techs along with their Advanced counterparts found lower down the Sub Cities Tech Tree add another significant boost to your Death-turned-Survival stat.
The combination of these three things will keep your sub Mayors alive through most every fight, often even after multiple losing reports!