The easiest way to defeat the Nightmare King Grimm

Charms: Hivehood, Sharp Shadow, Stalwart Shell (it help against the fireballs) and of course the grim child.

Tactics:
Basically, you need to hit him with your nail when he does Flame Pillar and Firebats. Use Sharp Shadow charm to damage him when he does Dive Dash and Dash Uppercut.

Good luck and sorry for the bad quality.

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