After selecting the ambition to end the greenskin invasion, on Expert/Expert, all I did was crush a couple of plethoras of orc young, and then (as I headed off towards the nearest strongholds, thinking we weren’t yet an elite force to be reckoned with, still short on heavy armour, greatswords and level 11+ members) a pop-up appeared to inform me that I’d already “beaten” the invasion. It never felt like a crisis. I had beaten some orcish bands that included modest amounts of warriors and the odd warlord shortly prior to that, as well as one or two combined greenskin hordes with an overseer and many ambushers and wolfriders, but I was expecting I’d have to overcome several waves of warriors/riders/ambushers/shamans before the crisis was averted. It was harder to end the noble war on Beginner mode, since there the mass of billhooks had destroyed my scale armours, and heavy crossbows had inflicted injuries that didn’t have time to heal, and I was trapped between two more hordes of c. 25 troops, and had to run and hide in the mountains in spare suits of light mail, running dangerously short on food supplies, and only “won” after narrowly escaping some roaming hordes of brigands, direwolves and weidergangers, which I led into the two noble forces, running away from the fight myself and only closing in to kill off the depleted noble force that remained afterwards.
The real late-game (the most fun time to be in the game with crowns to spare) still occurs later, around day 175, when you start to get a great many hordes of raiders and marksmen with several leaders and the aid of hedge knights or swordmasters or master archers. You also start to bump into more bounty hunters.