A couple things might help. I have found researching warfare and logistics early definitely helps in battling AI armies. Logistics lets you group your units (like squads) and that greatly pumps up your damage output and health. The AI will do this too, so if you don't, he'll crush you. Things get pretty easy once you start recruiting squads of archers.
Also, summon a few units. Early on, I get Rulgoth's Promise and the Ice Lord. The Ice Lord will kill everything.
Thirdly, be strategic in where you build cities. On my last map, I found a choke point and put a city there. I was then free to build anywhere on the other side of that point without fear of the other kingdoms coming after me. This means you don't have to spread your forces thin. Put the bulk of your forces in the choke point city, and a few defensive units in your others just to protect against wilderness creatures and bandits.
