Before revealing her Ankh, she gives you an opportunity to warp back to the surface and get back all your missing items after solving The Seven Gates of the Underworld if you pick the third option during your conversation with her. The battle should still be fairly simple without needing to do this, though.
Don't let the platforms rise you into the ceiling: You will get crushed and die.
In her panic state, she will try throwing her arm and spear at you. Either get up close if you have enough HP, or spam Flares from below until she's defeated.
Hel's third form has been buffed in the 220.127.116.11 update: She slashes faster and occasionally shoots a blue laser from the center of the floor.