Autobattle against Banshee (9 attack 0 armor and 30 hp) beat my entire army?

Always check for special abilities before entering battle, and never auto battle a Banshee. Learned that the hard way. I like that they are magic immune, feels all the better when I do take one down.

They are immune to physical damage and cast nasty damage spells so its quite possible it won, yes.

They are not magic immune, they are immune to all physical attacks and only magic can actually hurt them.

Realistically you would have to keep trying new attacks and keep guessing at how to hurt this thing before it killed you (trial and error). In this game you only have to read the caption. Life would be so much simpler if everyone just had a caption and I could access their instructions.

Er, yea, should have checked what I wrote, had magic on my brain I guess.

I did the same mistake with auto-battling Banshees once - I learned very quickly from that ;p

I both like and dislike things like this, mostly I like it.
- The good thing is that it forces you to use your head a bit - both when choosing to auto-battle, and when choosing how to gear your units.
- The bad thing is that even when you are overpowered you "must" still do things manually, just to make sure you don't get extra casualties.
On the other hand, such a risk is the price you pay for having things done for you.

just clicking on the banshee is enough to see that it's immune and nasty.

The good news is that it is a rare beast, nothing upgrades from its spawn point, and you can just deal with it when you need to.

Heart of Fire on a decent essence location is usually sufficient to take one down as it has very few hit points.  Counter Spelling it constantly is required.

the good news is - the banshee is really an exception. there are no other creatures in the world that are totally immune to physical damage. you'll make that mistake once, and remember it the next time just bring a champ with a damage spell or some weapon with bonus elemental damage -  or build a few guys with fire or frost staves or other enchanted weapons. if you bring mage to counterspell her wail, she's harmless.

there are other critters that are immune to certain types of damage (magic in general, or fire/frost/lightning/poison), but that's easy to overcome since most of your trained troops will deal non-magical, non-elemental damage anyway.

I don't think there's anything wrong with autobattle and banshees; it's more that their ACTUAL strength is not remotely represented by their raw stats.  So basically, Dark Wizard v2.0.

Banshees are easy.  One good flame dart and down they go.

Wouldn't the AI be getting constantly whacked when attacking banshees (since they always use auto-battle), or do they avoid them as a matter of course to let the player take care of them?

I hate them with a white hot hate, but I am actually quite impressed by how dangerous those little banshees work out to be.  You really have to think twice about those things!!