Is it better to have archers all target one unit or target individually?

For context, I’m playing a High Elves campaign with Imrik (first time as him, loving the campaign so far).

All of my armies have 5 sisters of Avelorn. When the enemy is approaching my line, is it more efficient to have the sisters all target the same unit, hopefully wiping them out before they make contact? Or, should they all target their own unit, whittling them away a little before engaging with my melee troops.

Typically, I have them target whatever monster/highest tier unit/lord in the hopes of destroying them before they reach my line.