I agree that the current method of defender choosing casualties is simple, so perhaps it should stay, but in war, the casualties were much more random, so here is my suggestion.
Custom create a Battle Board sheet. (A Microsoft Word table will do just fine.) Here is an example.
Battle Board
Attacker’s Units
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16
17 18 19 20
Defender’s Units
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16
17 18 19 20
Now, when a battle is declared, place the units on the board on a number in any order begining from the number 1 position.
Roll the Attacker’s dice as normal, for example there are 6 units defending, 4 Infantry on #'s 1-4, a Tank on #5, and a Fighter on #6. You get 3 hits, so now you roll 3 6-sided dice to see which units were hit. For example, you roll a 2, a 2, and a 6. Now you re-roll the second 2 and it is a 4, so the defender lost 2 Infantry and 1 Fighter.
You, the attacker, attacked with 4 Infantry, 3 Tanks, a Fighter, and a Bomber, so you place your units on the Attacker’s side #1-4=Infantry, 5-7=Tanks, 8=Fighter, 9=Bomber. The defender only hit twice, so he rolls 2 10-sided dice. (If he rolls a 10 or the same number, then he just re-rolls until he hits a valid target) He rolls a 5 and a 9, so he hits a Tank and a Bomber.
After casualties have been removed, then move the survivors down to the lowest numbers and use smaller-sided dice as needed.
Continue the battle until someone wins, or the attacker retreats.
Of course, you would need to have multi-sided dice such as, 4-sided, 6-sided, 8-sided, 10-sided, 12-sided, and 20-sided, but you can find these relatively easy at a hobby or gaming shop.
This system would seem to be fair, easy to use, and a bit more historical than the current system.
Of course there could be an occasion where you attack a fleet and happen to hit his 2 fully loaded carriers on the first turn, so you decide to retreat, which results in the 4 Fighters being lost at sea, but as we say, “Them’s the breaks!”
Using this system to assign Sub hits (to make sure that it’s not on Air units) would actually not be that difficult. You would just have to roll them separately, and if the Sub hits an Air unit, then just reroll the die until it hits a valid target.
What do y’all think?