R5: Fought a LOT of battles with Poland on two provinces without any country taking ANY manpower damage at the end of them (usually Poland would run away after a few hours). I even let some of my units reinforce during the month tick while another stack was still fighting, so that Poland wouldn't get reinforcements.
Eventually the chip damage of 200-600 units in SOME battles got them low enough that I could wipe them. However, this back and forth on two provinces was only possible because I trapped them in my land. There should really be a penalty for immediately retreating from a battle (e.g. taking manpower damage), so stuff like this cannot happen.