How much of a difference does the range make with Mist-like skills?

From what I've seen, monsters get poisoned in about 1 second, so unless the monster was especially fast, they'd get poisoned walking through a sling. I think that himes would be the only monster you'd train on that could run through it.

You wouldn't want to Sling at Himes because Himes are naturally poison-immune (I know it's just an example but I just wanted to point it out).