MHP HR2 Gathering Hall Urgent Quest: Facing Lao-Shan Lung (Hard)

I just can't seem to beat the Gathering Hall Hard rank Lao-Shan Lung.

Man, what should I do? I managed to defeat it in the Village Quests using the Master Blade Sword & Shield, but I wonder if that will cut it this time. Even back then, it was a barely-made-it close call. I'm properly aiming Pierce Ammo at the weak spot on its back, too. My son won't help me out either. Just when I thought about trying to control four players by myself, I saw a YouTuber doing exactly that.

I tried copying that 4-player setup, but it was just impossible for me...