You just do whatever works, if the only thing that works is forcefully holding her... then that is it until she understands that you mean business.
In my house brushing teeth was never optional - I think for 6 months or so I held down my DD mornings and nights to brush her teeth under screaming protest...having them scream in protest is actually easier than flat out refusal to open their mouths... at least they open up wide and you get done fast. Eventually my DD realized that this was a loosing battle for her and ever since she has been very good about letting me brush her teeth or now that she is older brush herself with me only helping occasionally.
Why am I such a hardass about it? I had and have TERRIBLE teeth, at DD's age I had cavities in pretty much every tooth and terrible bottle mouth - my mother never cared or taught us about oral hygiene... and my adult teeth are not much better... I would really love to spare my child a life of dental issues. In the end she does not remember our nightly teethbrushing struggle from early toddlerhood but so far we have not yet had a cavity.
Good luck.