Countess is always CI/FI but her minions are CI so you can kill them and then static countess while your merc kills her. The fallen demons might give you problems if there are a lot of them near her as they are FI.
I can't remember what FI's spawn in nihl runs, I know his minions are CI. I think he is FI sometimes but I may be wrong. I use a hammerdin so I don't pay as much attention while running him.
I don't really understand why people make light sorces for this if they are on a budget, as a fire sorc can do this much better. If you have "monsters rest in peace" mod on both you and your merc Nihla will be easy - for countess just kill minions, static, merc killz