Re: On using poison cloud units...
 

Enchantment 4 (or 5?), Nature 3(and 1 gem): Poison Ward.
50% more resistance to poison for all your troops on the battlefield. This stacks for C'tis - as long as you have Nature-3 mage with one nature gem with your armies you can stack all national C'tis units with all kinds of poison-critters.

50% resist does give them time, but it seems it most often it isn't worth the work needed to stack the casters with nature gems.