'Rust' Medic Sues Production, Says Fatal Shooting Was Traumatizing

The key medic on "Rust" is now taking legal action against Hannah Gutierrez-Reed and Dave Halls ... suing them over the fatal shooting that killed Halyna Hutchins.

Cherlyn Schaefer filed suit against the movie's armorer and assistant director -- plus the production company and others -- claiming the defendants' alleged negligence caused her severe emotional distress in the aftermath of it all.

According to the docs, obtained by TMZ, Schaefer claims she was the first one to provide medical assistance to Halyna after the shooting ... desperately fighting to save her life by applying pressure to her wounds, giving her oxygen and checking her vitals.

Schaefer also claims she was the one who called 911 and ordered a medical helicopter to airlift Halyna to the hospital ... where she eventually died.

In the suit, Schaefer claims she's since suffered "tremendous shock, trauma and severe emotional distress" ... and she's blaming the whole ordeal on the production's shortcomings with respect to safety issues.

Schaefer says she was regularly employed as a medic on film sets before "Rust" ... but now she claims she's so shaken she's medically prevented from returning to the profession or any other job.

Schaefer's suing Reed and Halls because she says they were responsible for Alec Baldwin being given the gun with a live round, which killed Halyna. Baldwin is not being sued.

She's also suing the movie's prop master, Sarah Zachry, claiming Zachry knew or should have known the gun contained live ammo.

Zachry's lawyer, William Waggoner, tells TMZ ... Zachry did not load the gun or handle the gun the day of the shooting, and "she did nothing wrong."

Schaefer is asking for unspecified damages.