I played around with the numbers, and came up with this:
Roll 1D6 in combat as before.
A unit which normally rolls 1D6-2 score a hit on a 6.
A unit which normally rolls an unmodified D6 scores a hit on a 4-6.
A unit which normally rolls a D6+2 scores a hit on a 2-5 and scores two hits on a 6.
If you get to double your hits (flank attacks) then roll two dice, and add together the hits scored.
If the hits are halved then roll a D6 for each hit scored; on a 4-6 that hit is ignored.
A unit is removed when it has taken four hits.
I tried this out in a couple of games, one of which you can see here: