When fighting her attack three times, if it was blocked roll away once and start tapping Square. If you do not hold any specific direction on the Analog-stick, just not touching it at all works. It will automatically target Whiptail. You can dodge her small wave attacks and use your Speed Stance to hack through the big waves to gain glyph ratings. If you do not press Circle to finish her, she will never die. Just keep attacking and dodging; and hacking through waves to get an easy three glyph rating.

During the fight against Whiptail, she sends out two different waves, Tidal Waves and Small Waves. You do not have to try and avoid these waves. Just hold L1 and press the Triangle for the Small Waves, and Square for the Tidal Waves. If timed correctly, you should slice through the waves and be able to run at a tired Whiptail.