Can we seperate the difficulties from the modifiers? Modifiers like Power Drain and Survivor are clearly handicaps disguised as difficulty levels. Not only do they add in extra effects, they increase the base difficulty on top of the active poisons.
Combine that with adding all modifiers to be selectible. So we can use whatever modifiers we want. Keep the multipliers for the different types of modifiers (increasing score for challenging, dividing score for helpful).