What's the best merc to pair with a Windy Druid in HC?

I've just picked up D2R, after hundreds/thousands of hours in D2 many years ago. Playing HC, I've just reached nightmare.

Looking around, there does seem to be a general consensus that the nightmare act 2 Holy Freeze merc is usually the best.

But does the effect of Holy Freeze stack with the cold effects of Hurricane? And even if it does, would I be better off diversifying a bit?

So far, I've found the Windy Druid's main threat is massed ranged enemies. Melee get slowed down by Hurricane and magic get absorbed by cyclone armor. So maybe Defiance is the better bet instead?