Tell the player: Yes, you can expect it to work. If it doesn't, that means that something else is going on.
As MC: Play generously, but still, say what honesty and your prep demand.
You'll notice a NON-1-for-1 correspondence between the Troll-Killer's equipment's powers and the example powers for monsters. The player will want to go to the troll and discover, imagine the luck!, that it is vulnerable to just the things they happen to have brought with them. Or, at worst, to something they happen to have seen hanging over someone's hearth. It's not your job to make this be so. It's fine with you if the troll really, truly is a terror to our children and our grandchildren.