I believe:

- Each soldier has his own experience
- It is increased by killing enemy...any enemy (except dogs)
- Each kill is worth the same. Quality of enemy doesnt matter.
- Router kill are worth 1/5th of non router kills
- Gaining levels gets harder. More kills needed for each level.

Merging units:

- I use my best troops to top up understrength units
- leaving small units of good troops which I retrain
- The retraining usually brings in new troops at the same exp as the good unit just retrained.
- Thus exp goes up quickly overall as all replacements via retraining come in at the highest exp level.