Most of the boss fights in TW1/2/3 follow the same pattern: On the first sight they seem to be very hard, maybe almost impossible. But they are constructed as a kind of riddle: Which combat techniques, which signs, oils, bombs ... fit best on the way to success. The beast is an instructive example.
Preparation:

1. The fight will begin at 0.00 - no matter when Geralt arrives in the cave. So potions and oils may expire if applied too early.
2. Meditate in the cave until 23 h. Apply Specter Oil. Geralt consumes swallow and tawny owl.
3. Leave the cave. Interrupt cutscenes with ESC, interrupt dialogue lines by clicking into the screen.
4. If your Aard is strong enough or you're lucky enough the fight will be done in some seconds:

https://www.youtube.com/watch?v=FAFgYQ0jGk0

If not:
5. Choose group style before leaving the cave.

It's important to keep Abigail on her feet: She's a healer, and a good one - as long as she is in the upright position for obvious reasons.
6. Stay near Abigail and kill the barguests first (group style!!!). If the barguests are eliminated then switch to strong style and attack the beast. Aard may be useful - but not necessary.

https://www.youtube.com/watch?v=TWzvem7mUsE

(DL: hard)

Btw.: Abigail will not die in this scene; she faints.

Good luck!