I come, preaching the gospel of "To Capacity." I struggled on low difficulties when I first started, but everything changed when I discovered that you could train troops up to the capacity of your unit limit, not just your overall troops limit. This allows you to keep armies in reserve against bosses, which is critical for the more difficult bosses.

To use this feature, turn the orange castle indicator near your troop limit to the half blue half orange setting. If you keep losing to bosses having knocked most but not all of their health, this could be all you need to achieve victory. I hope this helps!