I don't know how you could autoresolve this one to be honest. I did find this goal to be tricky - mainly because I tended to rely on diplomacy in the past to minimise my war fronts when playing normal legendary. Tried Iceni, Rome, Baktria and Sparta on this declare war on all setting before but was unsuccessful. now that I have done it I am sure I could do it in half the time now I know a little more.
Usually each new faction sends 4 stacks to around 8 stacks to you as soon as you get a border with them - but it is fairly manageable by that point. Once you get there it is all pretty easy as with other legendary. spent a lot of time working out how to expand out from my first three territories up to around 8-10. Admittedly it did take me 503 turns - but I finally got there.