Question for the vets.

[Grey_Fox]

I typically place my slingers on the far flanks of my army at a 45 degree angle to my infantry, facing inwards, which gives them good flanking fire. Just remember to cover them with cavalry.

[MarcusAureliusAntoninus]

For Roman missile units, I usually use the Celtic slingers in the north. They do real well.

[Thaatu]

Celtic slingers are the best (statwise) you'll find.