Well I am not much of a story writer but I would use third person as a narrator, because the other scenarios you suggest are still narrated in the first person, just from a distant point of view. I for one would find a story narrated in first person by someone outside the scene, like a bf watching her girl beaten or beating quite a turn off as far as I am concerned.
My idea is more like this:
Letizia was a 5'7 tall brunette with a body that, while not muscular, was well toned due to all her bellydancing. she wore her straight hair long and had brown eyes in a face both sweet and innocent. she looked younger than her 25 yo age.
Anna on the contrary was a relatively short black haired girl just out of her teens. At 20 yo she was little short of 5'5 and very slim, which was the envy of her former schoolmates in high school. She looked too skinny to be a fighter but she did have an attitude. Especially she loved to play dirty.
And dirty she did play. When the two girls met Anna spat out her hatred for Letizia: "I so fucking hate you belly dancer skank ! I will drop your body in a pile of trash when I am done with you!"
Letizia tried to punch Anna but the smaller girl blocked with her wrist. she was wearing black leathers and had chains on her wrists. The metal cut through Letizia's soft hand skin leaving her bloody bruises. Then Anna attacked. Her heavy metal boot flew up and connected with Letizia's soft womanhood sending a shockwave of devastating pain through the brunette's body. Letizia dropped on her knees and Anna started punching her face like a punch ball, with Letizia's head rocking left and right at each vicious blow. With a final uppercut to her nose Anna shattered Letizia's tender bone and the girl dropped on the pavement in a pool of her own blood dripping copious from her nostrils. Letizia was out cold. That was when Anna started having fun: "say bye to your washboard abs, skank!" she spat out before stomping her heavy boot again and again on Letizia's midsection . The poor girl's abs where soon hammered into a jelly and Anna's boots now sank deep into Letizia's tender flesh rearranging her insides. the pain woke Letizia up only for her to puke out blood from her mouth.
Satisfied with the destruction caused by her boots, Anna finally lifted Letizia's limp and broken body and admired how her guts were now a sorry mess, her once beautiful belly now horribly bruised and her washboard abs permanently crushed and destroyed.
Anna drag Letizia to a trashcan and, holding to her word, dropped the destroyed girl on a pile of trash before walking away fully satisfied .
There! That's my general idea