It seems to be kind of case to case. In my two games the AI barely resisted, in fact every single turn I gained ground simply from them retreating, no need o even engage in battles. The first game it was slow, about one hex per turn, plus the few I conquered, until they hunkered down in their only city, which I easily surrounded and took by repeated storming. The second time it was way easier, as the AI was also fighting another war on the opposite side of its borders and I literarilly marched my troops through their territory with nearly no resistance, and once I reached enemy troops they again started to retreat so fast my troops just barely kept up, until they ended in their single city as well (I had already taken the other two they mostly abandoned). I barely experienced any resistance except for the city garisons. And the AI had huge tech advantage as it was not even turn 30 yet, and they were nemesis, giving them more cities and tech at the start. Had they simply formed a defensive wall and not kept moving I might have seriously been unable to advance, or at least they might have had a chance. Did not have the will to try another game, as game turns get too long currently in late game, but dev said he plans to fix this, as well as many other improvements, and I am greatly looking towards those. There already were quite few changes from patch notes since then.
Minors never get better troops, so they are just there to be food the whole game, they only get more militia as game progresses.