Peacekeepers do strike quickly, but you just gotta pay close attention to the stances and indicators.
One tip I can give you, when a Peacekeeper does her lunge stab attack, it is always from the top stance. Always block from the top in that situation.
Also be very aware when they start getting close, chances are they will try grabbing you for that triple stab to inflict bleeding.
I think the best way to beat one is by being defensive, and parrying them. Also pay attention to their stamina bar, once they lose it go on the offensive!
As a Peacekeeper main, I personally have trouble with really defensive players. I can easily sidestep and avoid attacks from offensive players, and follow up with my own. Defensive not so much.
If someone can constantly block my attacks I'll lose my stamina real quick and I'll have to pull back for a bit. This is the perfect time to go offensive and get some hits in. Even parrying a Peacekeeper can almost guarantee you to take a major chunk of their health away.
Another devastating thing to do is to corner a Peacekeeper, removing their mobility forcing them into playing defensively. They can't make many mistakes or they're done for in that situation.