They tend to be fast, and have a long range for their attacks. They can easily drop your gladiators before you can reach them, so it is ill-advised to try to go one-on-one with an archer. Try to attack them with either units who have a very high DF score to dodge and tank the damage, or a Murmillo works well too, thanks to their innate abilities that cuts down projectile damage. By far the best way to take down an archer though, is to attack with either an Undead Legionnaire, or Taithleach the Undead Summoner. Either one will take single digit damage from their hits, and once you gain the Knit Bones healing ability, you can decimate them.If on YOUR side though, archers can prove to be a great help. Thanks to already having Eiji and probably a Peltist and/or Gungnir in your team, it is unlikely you will get one. If you do though, they have several decent abilities, like Jolt Arrow to stop distant enemies from completing their multi-turn move, cover area repeatedly damage a distant enemies shield, or Whistling Arrow, which will raise your crowd meter when there isn't anyone in range to shoot.