This same thing has happened with me too with the same chapter and same difficulty, though it was probably only 2 critical hits, which after the Laguz tiger kicked my Tauroneo's arse. After that, I restarted the chapter and took that damn resolve away from him.
I've had many hilarious/anti-climatic RNG events happening in my FE history, but can't remember most of them. One that has remained in my memory is from my first attempt on Awakening Lunatic+ (Where I used grinding at early game, and spotpass later on.)
I was in chapter 9 in a situation where I had to place Chrom x Frederick on certain tile to meat-shield incoming Wyvern Rider damage from weaker unit that couldn't take the blow. The Wyvern Rider had Counter, Pavise+ and hawkeye. I knew that the Wyvern Rider had counter, but because that Pavise+, I calculated that my Chrom couldn't die at the enemy phase, so I kept him equipped. For my inconvenience though, I had forgotten the Hawkeye completely and Tought it's already over. So what happened? The Wyvern Rider comes to haul his axe on Chrom's face but Frederick comes to dual guard him from the hit. Lucky! Then it's Chrom's turn. Too bad for me, Chrom says that Anything can change (even my calculations), Deals a Suicidal under 5% crit against the Counter and dies.