Don't add one shot mechanics, instead add more damage over time as is the case with Assembly and The Mage execute phases, this way dds won't afford to keep casting shields and dps at the same time and healers would be mandatory, this way people won't complain (as much) about the shield nerfs because they don't have to shield as often. Another idea would be making one shot mechanics have a longer windup before they actually one shot so players have more time to react.