Here's my opinion on some of the best long ranged fighters:
Eilie - This might possibly be personal preference. However, two free rune slots, a free weapon slot and all armor slots free isn't too shabby. None of her stats are all that high, but she isn't lacking in anything either. Basically, her stats are quite average. Suggested equipment is the Wind Hat, Winged Boots and Power Gloves x2.
Rina - What makes her one of the best has been stated already. The three free rune slots really contribute to this. Suggested equipment, probably the same as what Eilie is equipped with. I wouldn't bother much with magic runes, since her affinities are very average.
Kinnison - The same benefits as Eilie, except his technique is rather high. He can also equip the Master Garb, which is possibly the best piece of armor for pure damage dealers. Suggested equipment for Kinnison is once again, the Wind Hat, Winged Boots and Power Gloves x2.
When it comes to any damage dealer, strength and speed should be the number one concern, unless their technique is on the low side. Swapping one Power Gloves / Winged Boots for a Skill Ring might fix that. If you didn't know already, speed is responsible for characters attacking multiple enemies per turn. The higher the speed, the better the chance they have of accomplishing this.