[Ironfist]

It's mentioned in the GG that "shooting at enemy or being shot at by enemy" will also gain experience. From my experience, this works for relatively less experienced troops. When the troop is highly experienced, kill seems to be the most important factor.

My guess: the game records every unit's performance (shoot, being shot, causing casualty, kill, calling arty, etc.) in a battle. At the end of battle, it checks each unit's record and roll a dice. If the result is favorable, the unit gain experience from that part of record. If it is
a kill, the chance is higher. If the unit is not healthy (e.g., suffered heavy casualty in battle), the chance is lower. The result may also depends on current experience level of this unit. Just my guess.