I have also noticed some units gaining a chevron without dropping a single enemy. Perhaps just being there helps? I also had a medium phalanx slug it out with a massive amount of enemy infantry. It must have killed quite a few, but was reduced from 122 to 28. No chevron there.

In 213 BC, playing as Pontos, my best unit is a slinger unit at 3 silver chevrons. My best melee unit is a levy phalanx with 3 bronze chevrons.