In the famous charge of the 1st Minnesota at Gettysburg on July 2, there were 262 men in the regiment, with two companies on detached service.
215 of them were killed or wounded in just a few minutes.
That's 82.1%, a horrendous casualty rate. There was a Texas regiment or brigade (I've seen it listed both ways) that suffered an 82.3% casualty rate at Antietam/Sharpsburg.