Do you mean boss in first phase when she is giant? Run to a corner, duck, charge uppercut. If you are lucky, you can get her to get stuck in her charging grab. It can't hit you if you are ducking. You just have try and position yourself so she'll run past you while you are duck-charging. If she stops on you, you can expect to get grabbed unless you are super fast and run/jump away. You have brief invincibility during uppercut so try and uppercut through her as she is running past you, then duck again. If you can get the timing down, it's super easy, especially if you have all the upgrades at this point.
If you space properly, you can get her to run at you and stop behind you instead of on you. If you run TOO far away, she may leap at you. Run away if she stops on you. But once you figure out the spacing, just keep revving up those uppercut.