Gauss Weapons....Half the Armour, THEN apply the Armour Pen Value.
But, you know, if you don't want to go that route?
Stick n Shock! Half the armour, and then the troll makes his rolls based on how well the stick and shock hit Added fun?
Stick n Shock from a Machine Pistol! 8 Stun, Electric, + net hits against the the Troll's 10 armour + Body. Remember: you are
not rolling how much damage you inflicted, they are rolling to reduce that. If the Troll as 10 body, 20 armour, he is rolling 20
dice. If you got, say, 4 net hits on your shot using a burst fire on a gun using stick n shock, you would be at a damage value of
12 stun. So, he is going to have to roll really good to not take any damage from that...and you likely have a second 3 round burst already going at him...
Simply put: Stick n Shock: your answer for anything in armour that is not a vehicle.