I think you don't understand the question you're asking, or I'm misunderstanding your question.
Attacking a unit doesn't lessen it's attack strength, UNLESS you are using a unit with the "heat" skill. Considering how fighting works, there are far too many variables (abilities, terrain, boosts, base stats, AI's choices (which are predictable), and more). For example, having a choice between killing an artillery unit vs a heavy might make a difference (the heavy could do more damage on a lesser turn, but the artillery can attack units further back and almost anywhere on the map).
At the point where anyone really would start to care about testing this, they are already autobattling everything. Ultimately, attack strength is not directly related to health. My hover tank can take damage when on a non-plains tile, but wipe out the rest of the enemy after it moves onto plains, for example.
In practical terms, here's the best way to choose which units to take out. Choose the one that has the most imminent threat or will be threatening down the road. These include artillery that will destroy your rare unattached units (I hate plasma artillery when using hovers...), or heavies with a lot of health you will have to start cutting away (think Assault Tanks). However, you may want to leave some units with extra health and not kill them. Those units can flip your rogues for you on the cMap, and that's what you want.
Hope this helps a little and I'm not just rambling