He sets off the explosives after the first trolls pass the line of explosives; as such, a decent amount of troll parts are strewn over them, as the explosions rip through their line. Unfortunately, there's still quite a few trolls moving, the densely packed bodies having absorbed much of the blast, and the fact that the shrapnel wasn't strong enough to pierce more than one or two trolls, and in many cases, not fatally either, reducing the killing effect of his opening salvo. In all, he maybe destroyed about twenty, maybe twenty-five percent of the army, though many of them might recover, given time enough to find which leg is theirs.<br/><br/>The catapults certainly help, but the trolls are moving fairly quickly, and a few dozen get out of the killing zone and begins charging his line, while the remainder of the charge barrels through the oncoming fire salvo, moving more quickly than the shot can be reasonably reloaded.