Honestly, it says that my side is stronger and yet I die (especially it's a so stupid to die to a deer...). Is this some sort of joke? I know if I do it manually then I will win, but I simply don't want to combat is not fun for me at all. So what gives?

It probably depend alot of what types of units you use, like melee units may attack units behind walls and die, as well as the situation, things like terrain may play major role in how well automatic combat perform.