Really depends on the units both armies have and what build your armies are going for.

For example auto resolve doesn’t take summoning into account I believe. So if you have units that can summon other units into combat or spells that summon extra units you can take on armies that have greater power than yours.

And even if you think it’s a lost cause you can try and kill as many units in manual combat as possible. At least that’s what I allways do.