having individual levels of experience for each soldier in a unit is an interesting concept. However, there are two simpler ways of doing this. One way (similar to total war) is to have an experience pool that is divided among the soldiers of a unit, so in general the more battles won (and losses taken) the larger the experience gain/ level of the unit. Once a unit replenishes numbers the avg level of the unit goes down ... however the "survivors" tend to be the same people, so fight a 60 man team to 15 men twice, winning good battles, the first time 15 men have level 8 ... then replenish and the unit is level 2, then fight again ... and the 15 men (mostly the same ones) are now level 12. Replenish again, and the unit as a whole would be level 3.
An alternative would be to only have the "captain" of a unit, a symbolic leader, to keep the levels of the whole unit, and if the unit survives with even 1 person its assumed that the captain has survived, and you can replenish to Have full level. In this way 60 men fight down to 15 and acheive level 3 ... captain replenishes troops and fights again ... captain levels up to level 5.
The second would be a more slow level progression, as the idea is to keep small(ish) units that survive battles, as opposed to pumping more men into the unit ... has trade-offs that they fight better, but will also get killed off more quickly.
Another way of implementing the captain idea is to have the green troops slowly reach the captain's level ... say starting off at 25% captain level, and increasing another 25% (or 10%) for each successful battle. Close Fights however, say if the unit ends up killing hundreds of the enemy, the green troops could gain 50% or 75% (to max of captain level)
However ... the average troop level is probably the simplest idea, and while individual soldier level might be the most complicated (although in certain situations a sophisticated Captain-based system might be too complicated), the individual soldier level is also more "interesting" although the average level system is probably best for the computer, while at the same time tru to situationalized strengths.
Of course If I had to choose one or the other, I would rather have a sophisticated unit-based battle system (with many soldiers in each unit) than a sophisticated individual soldier system. (of course at the beginning when units are very small, it would give the illusion of an individual soldier system)