The Sky Dart is an extremely helpful skill for archers in Jeanne d’Arc as it allows them to attack from anywhere on the map. You can check out this Seeking Tech guide to see how you can acquire this ability.
To get this without heading into the post-game, you need to use skill binding.
The easiest way to do so is to combine the Meteor Bolt and Take Flight skill stones. You can get both of these in the Free Combat version of Tourelles, which is made available after you complete the 13th Stage of the campaign.
The female dark elves can drop Meteor Bolt. The Lizard Marshals can also drop Take Flight.
While you can also combine Mobility +1 with Take Flight, you won’t be able to get Mobility +1 stones until much later into the game.
Once you have the necessary ingredients, combine Meteor Bolt and Take Flight to get Sky Dart. Your archers will need to be at least level 30 to equip the skill.