Stephanie's ability—what Evenson had called the Force—was actually created when he repaired her body by infusing Gravitonium into her spine, granting her the power to control gravity.
Logically speaking, with such an ability, she needed no special preparation before battle. Anything on the battlefield could become her weapon. Not to mention naturally hard metals and rocks—even soft dirt and scattered grains of sand could become deadly weapons under the influence of gravity.
Take a coin, for example. With a light flick of her finger, even without the ability to generate electricity, she could alter gravity to give the coin tremendous acceleration and a stable trajectory, allowing it to unleash the power of an electromagnetic railgun.
And even if there was nothing at all on the battlefield, she could impose gravity directly upon her enemies. No need for anything excessive—ten times gravity was enough to kill an ordinary person. After all, not everyone had the physique of a Super Saiyan, capable of bouncing around under several hundred times gravity.
Still, being prepared was always better than being unprepared. Why scavenge materials on the battlefield when the things you had prepared yourself were easier to use? That was why she had brought such a huge mass of mercury with her to the battlefield. As for why she had chosen mercury? In Stephanie's somewhat childish way of thinking, mercury neither decayed, dried up, nor solidified, as though it symbolized an eternally beautiful state.
The more practical explanation was that mercury was poisonous. Extensive skin contact alone could poison someone, and since it was liquid, it was relatively easy to shape. On top of that, it was a heavy metal, so its weight was just right. To gather such a massive lump of mercury, she had smashed dozens of boxes of thermometers at home.