Supergirl has super strength which allows her to lift extremely heavy objects. She also has heat vision that can be used as a weapon. And of course, she can fly. Wonder Woman, on the other hand, has super strength as well, but she has unique weapons like the Lasso of Truth which can force people to tell the truth, and her indestructible bracelets which can deflect bullets.
Supergirl could potentially win. She has the same powers as Superman under a yellow sun. Her heat vision can be extremely powerful. If she uses her speed and heat vision in a coordinated way, she could overpower Wonder Woman. Also, Supergirl might be more adaptable in different environments as she has had to face various threats on Earth and in space.
The use of alternate universes is also popular. In an alternate universe, Supergirl could have different powers or a different personality, and the same goes for Doomsday. This allows for unique and unexpected storylines where the outcome of their battle is completely different from the mainstream comics or shows.
No. While 'Reign' is based on a true story, it takes many liberties for the sake of drama. It's a fictionalized account of Mary, Queen of Scots' reign, so not all events are portrayed exactly as they happened in history.
There are also fanfictions where the battle is a draw. Maybe they fight to a standstill, or some external factor intervenes, like another superhero coming in and stopping the fight before there's a clear winner. The beauty of fanfiction is that it can take different directions depending on the writer's creativity.
Supergirl is a popular character in fanfiction. Writers love to explore her powers, her dual identity, and her relationships. Her superhuman abilities make for exciting action scenes, and her kind heart makes her a beloved hero in many stories.