Elden Ring - Where is the Castle Morne Boss? (Leonine Misbegotten)

After delivering Irina's letter to her father Edgar at Castle Morne, something odd happened: Edgar doesn't want to leave the castle, because he needs to defend it while it's under siege. So how do we proceed from here? Very simple, let's help Edgar defeat the invaders.

So where is the boss? It's not that easy to find. Turns out that the portal to the boss is at the very bottom of the castle near the water, so you basically have to jump down all the way from the top of the castle to the bottom where the water is. Here is the progress, minute by minute:

0:00 "Behind the castle" site of grace
1:38 Stonesword Key
2:20 Pickled Turtle Neck
3:31 defeated by giant Misbegotten, after reviving fast travel to Church of Elleh to upgrade Lordsworn's Greatsword to +3
5:37 defeat giant Misbegotten
5:55 Obtain Twinblade Talisman from treasure chest
7:09 Obtain Tarnished Golden Sunflower
8:29 Obtain Smithing Stone
9:59 Obtain Whip
12:02 Use Wandering Noble Ashes to fight Castle Morne boss Leonine Misbegotten

After finding the boss Leonine Misbegotten, another issue occurred - even after summoning some help (wandering nobles), I can only get the boss's health down to about 70%. There is simply no way I can do it on my own. So what next?