Hi folks, it's been a while!
I have found Leliana to be one of the heaviest hitters in my party this playthrough. Currently at level 21.
First Specialization: Bard
Second Spec: Ranger
Strength: 20 (to wear the best light armour)
Dexterity: 35 (to get all the Archery talent trees + Evasion + best bows)
Willpower: enough to have Song of Courage + summoned beast and have some stamina left for a couple of tricks during battle
Cunning: max this out (make sure to take the Lethality rogue talent as early as possible, yes it works with ranged weapons).
Ignore the other stats.
Looking at her Heroic Accomplishments stats, her highest damage dealt was 549, which beats everyone else several times over. That's just with default ammo, I'm saving special ammo for when she really needs it.
Arrow of Slaying is a talent not a special ammo, yes it has a long cooldown, but used at the right moment it can be deadly. Scattershot doesn't do a great deal of damage to the main target, but is useful because it can briefly stun a clustered group of enemies - fling this into a group of distant archers to take the heat off your front line fighters. Add in the bard talents and a summoned animal to help, and Leliana is a force to be reckoned with.