Sry for this being towards the end of the current challenge modes but recently I was in a SoNT cm group and there is a way to prevent the bosses spin and be won't drop the debuff.,At the second boss, if you tank him at the top of the last stairs and all the range stand on top of the rock leaning against the wall there the boss won't do his charge or whirlwind and won't remove his stacks of the damage debuff.. I had to go searching for more details so I give some credit o raptorsounds on reddit. For reference this picture was made by raptorsounds https://i.imgur.com/O2jEBEj.jpg range stand on blue and melee and tank on red