Rust Mist question

[Borg]

Rust mist
Evocation lvl2
Earth 2
Water 1

Range 25
Area : 6+

Highly corrosive mist ......

So, I'm going up against some well protected troops and decide to script Rust Mist as the first spell to cast to try and lay the mist in between my troops and the enemy.
My Archers are firing and my melee troops are on "wait"
and what happens ...............

My mage casts another spell.
Then continues just as scripted.

Do the enemy targets have to be in range first ?
Can't you just lay out a mist about 25 in front of you ?

How about Sulphur Haze ?
Does that work in the same way then, needing targets first ?