Well what we Know
1. There are hit points
2. The number of hitpoints taken off a unit when it is hit depends on the strength of the other unit.
3. There is a random element (either in the chance of a unit hitting or how much damage it does when hit)
So Factors involved
Chance of hit: Random=? Strength=?
Damage per hit : Random=? Strength=Yes
Total unit base HP: Strength=?
Well 10 promotions would be hard to get (the shots we've seen seem to require increasing experience points per upgrade so 10 promotions would require over 50 experience at least (assuming 1 then 2 then 3, the slowest rate of increase),
Second, The promotions aren't That powerful. Now if it was a Warrior that you started the game with and kept upgrading to a Infantry... and accumulated 10 promotions on the way, then That might beat a Tank straight out of the factory.. but then it should (It could probably have not much more then a total of +200%, ie tripling, in any circumstance, but It would be an Infantry, and not just a x3 warrior which is only as strong as a phalanx so a tank Would crush it.