I did not say it was a good implementation, just that it had machanics to make choosing who would land the killing blow interesting. As I said, it depends on the type of game as well, and the system AoW had fit well with the type of game it was: one that focused on having elite, small forces and using them well.

Also some of its machanics would have made a per action system not work as well. See the resurgence ability.

And that is why it is interesting. There is risk, cost, and reward involved to make the desicion situational and not something to do without thought. Having the safest path always be the best is boring and leads to battles frequently falling out in exactly the same way.

Age of Wonders is more suited to making it interesting as its machanic make it so you think about more than than if your unit can kill the target and if the target might kill your unit. Specifically you have movement related considerations, where as in E:WoM most units do not have enough move to get in position and attack.

gain exp overtime... assuming there are special buildings or special bonuses that get applied once they are built, though your units have to be within your territory or in the confines of a city to gain the benefit.

Like a Jousting building, Swordsmanship School, Archery School, ect, ect. These would provide training bonuses of exp per turn to the units but be capped to a certain limit or level.

I'll grant that rewarding xp for something the player can easily manipulate - i.e. which unit gets the killing blow - does give you an extra risk vs. reward choice to consider, and makes combat that much more interesting. But there are many things that can be done to make combat more interesting, personally I'd prefer if manipulating which unit gets the xp wasn't one of them. As a gamer I'd rather be worrying about how to win combat, not who needs the xp - if winning isn't something you ever need to worry about and combat is therefore boring, that's certainly a problem, but there are other ways to fix it.

And this is entirely my opinion, but it also just breaks immersion for me; I picture Aragorn standing over the mortally wounded troll, raising his sword to deliver the death blow - and stopping as Frodo yells, "Wait! You're level 10 already, I need the xp!" Aragorn: "Oh, fine. See if you can hit it." Aragorn lowers his sword as Frodo stabs the troll in the shin, doing no appreciable damage. The troll makes a swipe at Aragorn, which he dodges. "Make it quick Frodo, I'm already down to half hp.."

Really I'd be satisfied with a simple xp split: if one unit kills a wolf in single combat he gets 10 xp, if 10 units kill a wolf they get 1 xp each. You'd still be able to level units that aren't especially good at killing things by letting them tag along, there'd just be a cost associated with it - the units actually doing the fighting would get less xp (as opposed to the current system, where there's no downside to bringing extra units, it's just free extra xp one would be a fool to pass up on). And once in combat there'd be no need to do anything unintuitive - like spamming an ability that grants xp or shuffling a weak unit to the front lines to get the killing blow - the player could just focus on the actual tactics of tactical combat without worrying about xp.

Assuming there are, at some point, actual tactics to consider. This would fall under the heading of "things that can be done to make combat more interesting" mentioned above - combat certainly needs something. I just don't think that something ought to be an xp system that can be manipulated during combat.

fixed your grammar errors

There are numerous problems with this type of system. Firstly, this system places heavy incentives on players to only keep parties of equivalent level. In the above scenario, by bringing units either too high or too low for the encounter, the player has lost out on 47 experience points out of the possible 100 xp. Secondly, this system places heavily incentives on players to not use lower level parties to take on high level monsters. Using your calculations from above, a party of 3 level 8 units would only gain 26.4 xp per unit upon victory instead of 33 xp per unit for a 3 level 10 unit party, even though the battle would be significantly harder. Thirdly, while this system is easy to describe using only one enemy of a particular level, large encounters with units of multiple level would require significantly more complex calculations, which ultimately would not reflect the true nature of the encounter. For instance, an encounter of 10 level 1 units versus 9 level 1 units and a level 20 unit would not significantly reflect the overwhelming advantage of the level 20 unit. Lastly, this system does not even attempt to reflect the relative strengths of units beyond their level. In your above scenario, you assume that the level 15 units is fair superior to a level 1 unit without addressing the possibility that the level 15 unit has a gnarled club and minimal armor and the level 1 unit has a lord hammer and heavy armor.