The problem is that, while your intention was to make them slower to kill, the reality is you made them impossible to kill.
It is actually impossible now since either the reflect damage received from the boss is too much, or we go defensively and our damage is not enough to counter the boss's HP regen.
So please have a look at this, and reduce the reflect damage from the bosses to make it realistic. Or the HP regen.
The defensive increase you did reduced our damage on the boss by x150. Im not exagurating, 150x times!