First of all you could try to use "AI - ignore guard position", sometimes this fixes ai problems for me, sometimes it doesn't. Honestly it's a complete mystery to me...
But other than that, the only thing you can do is to not use computer ai player afaik, i.e. have that player slot be empty/user.
That has the side-effect of completely removing all computer ai behaviors though, like for instance the units won't be using abilities unless they're on auto-cast or there's some trigger for it.