Soul of Sectonia in her form that is attached to the Dreamstalk typically pauses after every attack. To easily (but slowly) defeat her, use the Stone ability. During the brief pauses when Sectonia does not attack, use Heavy Smash near the small flowers with eyes. After you fall to the ground, stay in stone form and wait for Sectonia to finish attacking, then repeat the Heavy Smash on the flowers. Do not try to attack her when she is attacking; any risk-taking could cost life.