To be honest, I consider Armor Reducer totally useless.
Because when the Monster has full armor, the reducer takes great time and capacity to wipe it out, the actual health damage is usually less than 1 bar or even half.
Only when the armor is half full or less, the reducer achieve its purpose to hurt Monster instantly.
However if you take the time of using Armor Reducer to shot more Siren Missiles, it would be more effective.
I think TRS should either make the ability instant, or add some other use when Monster armor is gone.