Do you want the program to reduce strength for you or the players themselves by hand?
If the former, I would:
a) associate to the piece originating the hit a global key command with command key CTRL-A and matching property BasicName=$TargetName$
b) associate to the target piece a Dynamic Property HitsTaken with 0 as default and CTRL-A associated to “increment by” 1 (or CTRL-B to increment by -1)
c) associate to the target piece a bunch of triggers with matching property like “HitsTaken=1”, “HitsTaken=2” etc, all firing on CTRL-A or CTRL-B and triggering CTRL-1, CTRL-2 etc, respectively
d) associate to the target piece a layer trait “always active”, follow property value “HitsTaken”, and use layers to show hits left with level 0 being a label “8”, level 1 a label “6” etc (I suggest using Game Piece Images to do this quickly)
If the latter, then just skip step (a) and replace (b) to prompt players to select HitsTaken from a list, for example.