Pretty much anything that I let the computer auto-resolve when I had any weakened units in the first 6 slots or so.

Hint: Front load your heaviest infantry with the most troops first, then your healthy units, then your weak ones. otherwise the computer basically marches them single file into oblivion and routs your entire army.

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Once tried to take Krakow with a general and some minor militia men while on crusade to take the long campaign in 8 turns, only to have been ambushed with their mounted javelin throwers and their king.

I managed to slay their king and rout 90 percent of their troops, but being so very severely outmanned, their routing troops just kept coming back over and over, slamming into me from all directions and peppering the remnants of my troops with javelins and arrows. Couldn't rout them all, and my general was among the last to be slain. The battle ended one second later.

Almost all my troops were destroyed, and since I only managed to rout their armies, they pretty much survived. Cost me my 8 turn victory, and there was no way that redoing the battle would have won it. I tried. Repeatedly.

I just needed to commit more troops, that's all. After several valliant struggles, numbers eventually crushed me. I'm not immortal after all. Just practically immortal.