Reimu's bean took a direct hit. And Marisa probably saw the walking artillery arrive and found enough reason to skidaddle. Or she's out comforting Reimu's bean. One of the two.
This image makes me realize something: As epically Grand and Charismatic a presence as Imizu's Remilia commands (especially in her comic Vs Reisen), she's still a pipsqueak in terms of stature. xD