If you manage to get a duo on GC and get Civvie Defense, the best way to handle it is two ballistae and a barricade or two. You train the ballistae to both hit the same spot and give them speed and damage, and drop the barricades at the hit area to slow things down - just in case things don't go just as planned. (I haven't tested, but I assume if you put barricades irrationally sideways, you get a longer slowdown effect as mobs run down the length of the effect zone instead of across.) Then you run to spam tripwires at the hit area while the trash rolls in every time it is off cooldown. The trolls take four shots to kill, which is two shots a ballistae, which you can do with ease when you have tripwire maxed out. And there's no reason why you shouldn't, it is one of the most effective things Engineer line gives you.
It is not a good idea to let mobs near their targets. This applies to several quests, such as that tower defense in the Dike where your best bet is to lock everything down at the choke point with three lines of barricades. You don't even need to be there to assist, the guards will kill everyone when in two handed stance.