Splice

Genetic engineers Clive Nicoli and Elsa Kast hope to achieve fame by splicing animal DNA to create hybrids for medical use at the company N.E.R.D. (short for Nucleic Exchange Research and Development). Their work previously yielded Fred, a dog-sized vermiform creature intended as a mate for their female specimen, Ginger. After successfully mating them, Clive and Elsa plan to create a human-animal hybrid that could revolutionize science. Their employers Joan Chorot of N.E.R.D. and William Barlow forbid them to do this. Instead, they are to find and extract proteins used for commercial drug production from Fred and Ginger. Clive and Elsa pursue their own agenda in secret. They develop a viable prepubescent female creature.
Although they had planned to terminate the hybrid before it reached full term, Elsa persuades Clive to let it live. The hybrid subsequently becomes aggressive and stings Elsa several times. The hybrid sheds body parts in an effort to escape when they try to destroy her, but they subdue her. They discover that she is aging at a vastly accelerated rate. Elsa discovers that the creature is undergoing mental development such as that of a young human child. Elsa names the creature "Dren" after the creature spells out NERD, having seen the letters on Elsa's shirt. Elsa subsequently refuses to let Clive refer to her as a "specimen".
After moving Dren to a new location for fear of discovery, they find she has a dangerously high fever. In an attempt to save her they place her in a large industrial sink full of cold water. Clive places a hand around Dren's neck and pushes her under the water, seemingly drowning her. However, it is found that Dren is amphibious, though it is unclear whether Clive knew this, having analysed scans of Dren, or whether he did intend to kill her.
While studying Dren, Elsa and Clive neglect their work with Fred and Ginger. At a highly publicized presentation of their work, Fred and Ginger savagely fight to the death. It is subsequently discovered that Ginger had spontaneously changed to a male, but Elsa and Clive failed to notice because they were focused on Dren.
Elsa forms a motherly bond with Dren. After Dren attacks Clive's brother, Gavin, they move her to an isolated farm. There, Dren develops carnivorous tendencies and retractable wings. She grows into adolescence and becomes bored with being locked up in the barn, but Elsa and Clive fear that letting her outside might lead to her discovery. Clive realizes that the human DNA used to make Dren was Elsa's, not from an anonymous donor as Elsa had told him. When Dren assaults Elsa again, Elsa removes Dren's stinger and uses it to synthesize the protein for which they had been searching.
Dren seduces Clive; Elsa discovers them having sex in the barn and becomes upset. Clive accuses Elsa of never wanting a "normal" child because of her fear of losing control; instead she chose to raise one as an experiment, where control could be assured. Deciding the only solution is to terminate Dren, they return to the farm and find Dren already dying.
William Barlow discovers human DNA in Dren's protein samples and arrives to investigate. Elsa tells Barlow that Dren is dead and buried behind the barn. However, a winged, male Dren rises from the grave and attacks the group, killing Barlow and Gavin. The male Dren rapes Elsa, before Clive attacks him. Dren overwhelms Clive, when Elsa attacks with a large rock. Elsa then hesitates, allowing Dren to kill Clive before Elsa finally kills her creation.
Elsa is later informed that Dren's body contained numerous biochemical compounds for which the company has begun filing patents. Joan offers Elsa, now visibly pregnant with Dren's baby, a large sum of money to go through with the pregnancy, which Elsa accepts.

Question: Who attacks with a large rock?
Answer: Elsa.