Follow the player with two locations, 19x12 and one that is like 24x17.
Assuming you're killing CPU controlled players that are owned by Player 8, here's some triggers you can use:
Give all men owned by P8 at inner location to P9
Kill all men owned by P8 at outer location
Give all men owned by P9 at inner location to P8
This would have the disadvantage that because you are constantly giving units, players wouldn't be able to select them, so here's some extra triggers you can add so that you only need to carry out the killing action when required (at that instant you would lose selection of any units):
The best way to carry this out is to store the number of units in each location into a death-count. Like:
Player brings 1 men to inner location, set InnerDC to 1
Player brings 2 men to inner location, set InnerDC to 2
Player brings 3 men to inner location, set InnerDC to 3
...
Player brings 20 men to inner location, set InnerDC to 20
Player brings 1 men to outer location, set OuterDC to 1
Player brings 2 men to outer location, set OuterDC to 2
...
Player brings 20 men to outer location, set OuterDC to 20
Then have triggers to compare the DCs using binary arithmetic:
If InnerDC at least 8, subtract 8 from InnerDC, subtract 8 from OuterDC
If InnerDC at least 4, subtract 4 from InnerDC, subtract 4 from OuterDC
If InnerDC at least 2, subtract 2 from InnerDC, subtract 2 from OuterDC
If InnerDC at least 1, subtract 1 from InnerDC, subtract 1 from OuterDC
If InnerDC is exactly 0 and OuterDC is at least 1 -> we have a man in the outer location that is not present in the inner location!
None.