Neversofew615219, Units do not simply die for no reason.
The problem comes from you having a unit in your army that takes away defense from all other units in your army. An example of this is the Apocalypse Horse. It takes away 8 defense from everything else in your army. I had this problem once, but I figured out that it was my Apocalypse Horse causing it, so I took it out of my army. Scan through all of your units (In your army), and all of the equipment that you have on your units (In your army). If anything takes away defense from your units, take it off and swap it for something that doesn’t. Then test your army in survival and watch what happens.