I fought the same 3 rat armies 8 times and killed 17 rat lords in one end turn.

A recent patch added a feature/bug that when a lord is killed and replaced by a new one, that new one seems to get their full movement range back. This combined with having only spirit leech as my damage spell meant I could only kill the enemy lords with ease. The rest of the rats mostly ran away, meaning they didn't take many casualties and could get a new lord who could attack again.

This started with Skrolk attacking me, he died and then he brought in 2 other armies. My lord has the Clownfather Nurgle trait which was a huge reason the leadership bombing worked. The area damage from that trait was also the only aoe damage we had and it's the reason I'm travelling with only 2 lords and 1 hero. The trait is that strong.

I also had a Mist Mage with the healing overcast spell. This let my lords and hero stay at full health between each battle. And the Chracian Scout on my lord made it so only the very last battle was an ambush. I got lucky in that regard, because she only got her horse right before the final battle.

In total I killed 17 lords and 1 hero over the course of 8 battles. It took 2 hours of pure grind without Purple Sun.