When Harley is using her continuous (special) attacks she is invulnerable. The player should run away from until she becomes "dizzy".
When the player gets the message that Harley is "dizzy" (hammer spin) or smashed her toe (repeated hammer slam), the player gets a few free hits.
Some form of healing is crucial. A player can run back to where he fought Lefty and Righty (where you dropped down). Three Health Barrels (which respawn rapidly) are located there. Harley will follow you down.
Harley does not use her grounding attack when she is in either of her continuous (special) attack modes. For fliers, it is recommended they fly upwards the moment grounding effect wears off (dodging before that). This would actually make those periods completely safe. However, do not hover idle - while she is immune to damage at those times, attacking her from a distance is still a good idea as it helps build Supercharge.