Cni's a rare critter, at least. Conceptually. I was blast mining my way around the underground desert (revengence makes that a lot more sustainable early on, by the by) and ran into like a dozen of the ruddy things inside maybe 20, 30 minutes at the most.
Best suggestion I have with it is to have some kind of minion that will track onto things (brier will do it, I'm currently using meteor heads because they're friggin' amazing, especially with the meteor armor summoning set) and then hide behind a wall. The projectiles don't pierce, so as long as it's not cheerfully ignoring collision you're pretty much entirely safe as your minions whittle down their health.
E: Huh. Well that's interesting. Dunno if it's intentional, but once you get access to the enchanter, you can enchant weapons. Oddity to it, the effect isn't bound to the specific weapon's attacks, so if you have, say, the poison cloud proc one on your primary, any other sources of damage you're putting out also gain the effect so long as you're currently using the enchanted weapon.
Sources like, say, minions. Because these meteor heads totally needed extra crit, and a poison proc that puts out four clouds each doing like 4 times a single head hit, on top of the meteor armor's "Hey let's juggle that darksteel knight" summoner set ability. Whee.