So how did St. Olaf massacre the trolls?

During the film we see the skeletal remains of the troll's kin in one scene, and are informed that the creature hates Christians so fiercely, because St. Olaf, in converting Norway to Christianity, rid the land of everything that was antithetical to the new faith, and so, he massacred the trolls, even the troll children. The problem with this is, how? In an earlier scene, we see the troll shrug off hits from tank guns and missiles, any damage inflicted healing almost instantaneously. The only thing shown to be able to harm the troll is ultraviolet light, and sunlight turns them to stone, which is not what happened to the dead trolls whose remains are shown in that scene.

So how to the Vikings of St. Olaf's time, armed only with bows, swords, axes, and spears, take out creatures that can shrug off damage from hellfire missiles and 120mm tank guns?