Enchantress with dual cresent moon dream merc?

Odds are it'd work but be quite underwhelming.

You'd get top-end attack speed out of it to combine with your

Enchant and Infinity so that's pretty much the ideal starting point.

Beyond that though, you'd want some way to handle fire immunes as a sunder on you won't work for your enchanted merc.
Going with Dream + crescents would likely work but it wouldn't exactly be great, both as additional dmg on non-immunes as well as sole dmg on fire immunes. The -res on crescent would work to improve his Dream dmg but with no mastery and no synergies it starts at 1-648 added which even after -res still isn't exactly massive. Further, since you mentioned that the one thing you would do besides enchanting is using static, the static procs on crescents will effectively be pointless as you'll have long since taken care of that.

I dare say you're probably far better off using a solid physical approach added to

Enchant instead. I.e. the trusty ol' double Grief. Not only would that offer far more damage overall but maybe more importantly, it'd offer far more physical damage, meaning your merc can also get far more leech and hence survivability.

Alternatively, to further focus on the

Enchant aspect, give him a Plague for lower res to bump up that damage even more. Though that will of course mean you're sacrificing performance on fire immunes to do so. Give and take.