Look at this:
Firestorm, 5
, offers -0/-3 on all enemy units.
Lightning Storm, 2
, offers -0/-2 on all enemy units.
Improved Plague, 3
, offers 1 poison counter on all enemy units.
Desiccation, 4
, offers -0/-2 on all enemy units plus 2
for every unit damaged.
My only incentive to use this over any of the above would be the Attack reduction. Even if it can go into the negatives, I'm not thinking of using this at its current price. And that's upped.
Unupped, it fares
strictly worse than any of the unupped counterpart of the four stuffs I listed. (Well, maybe not Dry Spell, depending on the sturdiness of your units.)
Also, Entropy already has its share of mass CC with Pandemonium.
(And, IMHO, this effect feels more at home at Death or Time than at Entropy.)
A little nitpick: Losing -1/-1 would be actually
gaining +1/+1.
Also, does spell comes with an ability? I'm suggesting these:
-)
All enemy creatures gets -1/-1-) Scale your costs (and maybe the element)