Mmm... i don't think in a big battle you can do much to improve your chances.
You must have to be lucky when you destroy or nearly destroy an enemy stack. Supply wagons are the most easily captured units, as they don't really fight in the battle.
The easiest way to get a nice free light gun unit would be to find some lone battery being translated to the front or a small stack that you can overcome with little combat.
But thats easier said than done
