To the extent that it is possible to be prepared for fighting a landwurm, Sigyn isn't. He brought his hunting bow, a short sword, and miscellaneous knives. Without a perfect opportunity, which this isn't, the bow won't do much more than annoy it. The sword isn't all that much better, especially considering how close to the wurm he'd have to get in order to use it. In short, trying to engage would be utter foolishness.

But here is Loki, who doesn't want to be eaten by a landwurm either, and in fact probably holds this preference more strongly than Sigyn does, and who is currently in midair above its lunging jaws.

This logic comes to his mind all at once, not in an orderly chain but as a unified vision, and he draws his sword and hurls himself at the monster's head. He manages to get between it and Loki. Well done, Sigyn. Now the next step in the plan is to survive its redirected attention and, if he is very very lucky, stab it in the eye before it eats him.