Don't have damage deduction stack. Abilities can stack but damage reduction might be overkill.
How so...?
Well stacking 2 sheilds that reduce damage by 3 for a total of 6 seems a little insane to me, but you know what is more insane? Em shield to reflect and reduce damage by 2 with permafrost shied to make creatures into icecubes, and have a total damage reduction of 4 while you are at it.
Even if damage don't stack there some fairly broken combos heres.
Thorn and dusk mantle. 50% of attacks miss, and because they still attacked they can still get poisoned.
I still think this card is a good idea if certain limits are placed on it