You are in the "terrible twos!" and you are locked in a power struggle with her. Next time it is tooth brush time time try giving her the tooth brush with tooth paste and walking away. This is the opposite of what she expects.
You are removing your attention from her, just the opposite of what she wants. She wants you locked in the bathroom with her. Don't be surprised if she screams at you to come back, or runs after you and hold on to you clothing to keep you in the room.
Tell her you will only come back if she stops screaming and will calmly let you help in the tooth brushing. You might have to do this a number of times to reinforce this.
Stay calm, if you feel like screaming go out side, lock the outside door so she can't get out, and scream. (I used the shower.)
You will need to choose your battles during this year--some battles are not worth fighting, so don't even begin. The battles you choose to fight, fight to win and don't give up.