For those (like me) who had a little trouble beating Boudica, here is my strategy. I played on the hardest starting difficulty.
First, she will block or evade anything you throw at her so the only way i have found to beat her is countering (and patience).
She attacks with three strikes that you need to deflect with the A button. At the end of these deflections, if Boudica staggers back counter with X-X-Y. This will give you three hits on her. Do not be tempted to go for more, she will recover and counter you. It's important to get into the correct rhythm here, just experiment.
If Boudica does not stagger after your deflections just wait for the next round of attacks from her and repeat. Be sure to leave yourself with room to maneuver so you can roll out of the way if she comes in with her heavy attack (she flashes red like all the other heavy enemies). She comes in with the heavy attack both after her three hits and on its own. When she comes in with her heavy attack after her three hits you can counter with Y if you are quick, or roll out of the way with B. I found it easiest to use Y when she uses the heavy as part of her normal attack and and B when she only comes in with the heavy attack.
After you beat Boudica you will need to take care of her guards as well before the game will save so make sure to keep your health up. I had to take care of two axemen and three heavies with shields, not sure if that varies with difficulty.
This was the first difficult fight of the game for me, took me a few tries.