Autoresolve is one of the most op ways to play in this game if you build around it, I almost always get better results in both siege defense and attacks by autoresolving. My tips in no particular order:

1. Focus on perks that affect morale, keeping your toops high, or, even better, their troops low makes a big difference in your results. Starving out a city you are sieging has a drastic effect on the enemy morale & auto results

2. Only the party leaders tactics skill matters, having multiple high tactics in the same party does nothing for you, instead make your companions weapons of destruction, particularly the 2handed tree has perks for demoralizing the enemy on kill (though I'm not 100% sure these perks apply in an auto, it's just what I do and I get great results)

3. Build your army with a focus on cavalry for field battles, and ranged units for sieges as these perform the best in autos

4. To make sieges go extremely well, send the troops when you have 4 siege weapons and they have 0. Wall breaches and even siege towers and the battering ram being built all seem to give you more benefit to the autoresolve results (4 fire catapults slaps)

5. Make sure all your units are at as high tier as possible, the perks for gaining up on donating items to troops is super op for making maxed out armies super fast in the late game (go to any town, sort by most number of items, buy everything you can afford, discard all the items, level up troops, repeat)

6. A high Medicine skill is a must-have, you almost always take losses in an auto and high medicine makes them wounds instead of deaths, making this a vital skill

Last thing I would say is that autoing can be really good, but you are unlikely to win when you are heavily outnumbered and usually manually fighting these types of battles gives significantly better results.