First question is what are my chances with just 2 TrebA's, 1 ByzInf, 1 Spear, 1UM, 80 Szekelys (??? some Merc cav unit, with axes) and 20 Kat'oi against that lot?
depends on their morale/valor. Low valor ByzInf will break when flanked, higher valor might hold a while. So what's the generals command rank? Does he have valor adding virtues? What's the terrain? TBH, it looks bad.

I abandon when doing so doesn't create a strategic problem (like letting a large enemy force have a province that would allow them access to my creamy soft interior provinces, and when the troops I would save are more important than the chance to kill lots of enemy (IMO its easier to kill wagonloads when defending). Or when trying to hold is just plain unreasonable.

ichi