Do not bring armour as it will not protect you from the attacks. However, either Warpriest of Saradomin armour or Warpriest of Zamorak armour has a chance to reduce damage taken from the Beast's attacks, so it's recommended to bring all the pieces you have (the more, higher the chance).
Enemies to defeat
Luring the beast
Talk to Summer at the wilderness wall, north-east of Varrock. (1)…
Okay, I'll come.
Enter the portal. (~)…
After the dialogue, operate Jennica's ring while standing on the portal.
Run north to the dungeon, collapse the entrance.
Return to the spirit realm.
Run north and lure the beast to the entrance with your blue charm familiar. Enter the cave before it has a chance to consume your familiar. (1)…
Yes, I am.
Inside the cave
Avoid the beast's ranged attacks by moving away from the missiles.
Run north and cut 15 logs from the wall.
Once you have 15, light 3 fire pits (5 each). (Right-click use logs on fire pits)
(Optional) Restock your food and return to Summer to continue.
The dark core
Enter the next room by going through the passage in the south-east corner. (1)…
Yes, I know what to do.
Avoid the missiles like before.
Soon a dark core will appear, dig a hole in the soil and lure the core into it.
Repeat this 3 times until you receive a cutscene.
The coloured graves
Watch carefully which graves the coloured spirits enter.
The beast will charge a coloured attack, while standing behind a grave using it as a shield, bless the grave with the matching colour.