Ah, I think I can guess the other half of the secret.
You don't retreat from the battle, right?
I tried it and it worked quite well. The retreating cost me some precious turns of going into the same province I cleared to get to other provinces though. Was on the other hand on a very tight spot in the map, on a open map it might work quite well. And starting to pull in about 100 blood slaves per turn mid year 2 without having focused on building a blood economy is nice.
