You run for the gun, and you catch Mary Lynn off guard. Smugly reflecting on her new level of authority, she lost herself in her daydreams of what she would do with unlimited power, and she didn't even notice you were making a move until you already got to the gun. Well done on your part.
You also receive an unbelievably lucky break, when it turns out the gun has a feature you never knew about: it changes its own size to fit in the hand of whoever holds it. What a neat invention! After a quick stumble, you grab the gun in your hand. Unfortunately, by the time you do, Mary Lynn has regained her senses and wrapped her fist back around you. Even if you've got the gun, she's still the giantess. She pulls you back up towards herself, and you know you've only got a split-second to figure out what to do.
What should you use the gun to do!