Thor is usually the epitome strength and skill in any story that he's in. But this story is a little different than most, because Loki tricks Thor out of carrying his hammer or wearing his girdle. You might see Thor's hammer and girdle as the source of his strength and skill, and they do have magical properties that make Thor pretty much invincible. But actually, Thor does just fine here—thank you very much—thanks to Grid's sage advice and gifts of important (though non-magical) objects.
Turns out that Thor's strength and skill don't stay home just because his hammer and girdle do. And Geirrod seems to realize it. Even though he knows that Thor is weaponless and armorless, he still resorts to trickery in his attempt to kill the god.