It was the fight with Zohakuten, Mitsuri was holding her own against his wooden dragon heads as Tanjiro, Nezuko, and Genya hunted down Hantengu, trying to slice his head off

Mitsuri was getting tired, very tired, she was close to failing and with a wound on her forehead that leaked blood down onto the bride of he nose, perspiration coming from her body as she felt herself struggling, and her hair braids where slightly dishevaled and she was missing her haori, she was fighting and fighting, swinging her softblade around like a ballerina with a spinning silk ribbon of pink and black metal. Then she finally caved, she let out a scream of frustration and sadness, clutching the hilt of her sword tightly and close to her to her chest as tears started welling up in her eyes I can't take it anymore!! She shouted, giving one final slash before a wooden dragon head lunged at her, mouth agape about to swallow her whole as time slowed down for a few seconds, this might be her last few living moments