What’s the one shot tech with hunting?

I’m trying to farm chill shots for money and Agatha contest and I heard that using a wither skeleton pet or crimson armor makes it to where you can one shot them but neither works. With the crimson armor they have 50 health left which isn’t 10% so they can’t get black holed. The wither skeleton on the other hand just doesn’t work the decay ability just doesn’t activate. I’ve tried tuning my stats so that I do around 360-390 but every once in a while I just randomly do 450 damage and instakill them. I need to know the one shot tech or learn how to stop randomly one shotting them.