This method is even faster than waiting for the rematch after might, skill, and will training.
During the very first fight with Whisper when you conduct block and roll training, use the third part of the training to get the achievement. Thunder will walk up and the Guildmaster will tell you to combine attack and defense. Whisper will get a health bar and the fight begins.
Hold block until she gets close. Try to roll slightly diagonal toward her so that you end up behind her. She will typically attack twice in a row. Wait for both strikes while blocking, then roll behind her after the second and start hitting her back straight on. If you are right behind her, you can sustain your attack until her health bar runs out. Don't use the flourish attack or you will knock her down and have to wait for her to get up. If you do knock her down, be sure to wait behind her so that you can start assaulting her again. Once she is out of health, the achievement will pop immediately if you were not damaged.