You will be trapped in a room with Agatha and all the doors are locked.
She Teleports a few times (around 5-7) and then chases you.

This boss battle only takes about 2 1/2- 3 minutes. She has about 5-6 teleport/run after you cycles. It feels like eternity, but it's really only a few minutes. This thought is what kept me from giving up.

-Get into a corner and look into the room when she starts teleporting. Preferably a corner not next to the stage. This gives you a good view of where she is at.

-When she starts coming for you, start running away and wait a bit before using your boost. (you want the boost last as long as possible.) Unless she is right next to you of course.

-Run around the parameter of the room while avoiding the nobs on the doors, they will slow you down. Eventually she will start teleporting again. Get back into a corner and observe.

-Eventually she will start throwing dummy portals out. While you are boosting away from her run through them and they will explode. YOU WANT TO DO THIS. DUE TO A GLITCH YOU CAN DIE FROM THESE EXPLODING WHILE IN A CUTSCENE AND YOU WILL HAVE TO REDO THE BOSS FIGHT. By the time Bierce opens the door out to the hallway, you want to have all the portals gone to avoid this glitch. *Note: a reader pointed out to me that they had to keep the portals there in order to not die. So if you are having issues on this step, try that instead. :)*

-Do not trigger the dummy portals while walking. (I haven't tried this, but you might die.)

-After you avoid Agatha for a few rounds, the doors will open to the hallway and a cutscene of Agatha plays.

-After the cutscene you will still be looking at Agatha. She will run towards you and kill you, turn around immediately (middle mouse click) and run through the hallway with a boost and avoid the bus blocking part of the hall.

Congrats you did it! I can't tell you how many times I tried to beat this boss fight. If I had known these tips it would've gone much faster.

-In the second round of Agatha Teleporting, she will do it much shorter (around 5). Be on your guard for her running towards you.

-I had my boost ability upgraded at least once beyond the base stats. It's probably easier if you upgrade it even more. *The Steam user masquerade in the comments below recommends Boost level 3*