How can I handle a bossy coworker?

Bullies only back down when you stand up to them. In a professional setting, this means guarding your time and/or appealing to a higher power (your supervisor) to help "referee" the matter.

Ignore them

If they are not one of the chosen ones, just ignore them & do the best you can. If they are one of the untouchables that do & say whatever they wish (racist, bigoted, hate speech, sexist, misogynist, backstabber, liar, gossip, etc...) and are protected, then it is best to be as polite as possible & do what you are told as long as it isn't against your actual boss, management, company or gaming rules/laws. Otherwise, you are welcoming threats, assault (verbal, physical, at as well as away from work,) and everything you say or do is put under a microscope for review & reprimand, while they continue to attack & threaten you.

Personally I would punch them.in the throat... Professionally, you report them to the manager, you only have one boss and seniority no longer exists in the work place.

I'm direct so I work with it head on.

Explain that you have your own work to do and that you do not have resources to do their work as well.

Let them know what you ARE working on now, and tell her what you CAN help with (if applicable).