You Propably forgot the dialog and to press "use skill" on the Dark Knight which 1/5 Damage dealed by enemys. Making the big demons Attack go down from 5000 to 1000 for 2 minutes.
Nothing we have can tank that 5k damage without the skill of the dark knight. Not even 99AW Gellius during skill use with a dancer and AW Bernice... maaaybe able to survive one hit and thats it cause that big monster attacks pretty fast.
EDIT: Just saw ya already got an answer lol