Illusions were not a favorite of Sif's. When she could not trust what her senses were telling her, she could not anticipate cause and reaction; brute strength, weapons expertise, and strategy all became unpredictable and somewhat futile. But everyone had their weaknesses.
Right now and most every other time, the goddess played to her strengths. Before Thor's words even left his throat, she was on the move, keeping sharp tabs on where the others were in comparison to the beast. She moved near it's hind haunches, blades drawn and ready to slice through tendon and cripple the thing, should it hold long enough to shake off Mjolnir's blow.