Get A Character To Zero Health And Survive 5 Attacks, Not Required For Them To Be In A Row, You Can Heal Inbetween, But Every Hit Lowers Your Survival Chance For the Next Hit.
You Can Equip Certain Heirlooms To Increase Your Chance To Survive A Hit While At Deaths Door, But In The End It's Still Pretty Luck Based.
I Wouldn't Go For This Achievement Unless Its The Last One You Need, As You Might Be Lucky Enough To Get It While Playing.
Good Luck!