If you're playing on Normal, those archers probably didn't have an equipped Bow, which is necessary for effective damage against fliers. On Hard, equipped Bows are basically on every single archer, so it doesn't go as well, but Pegs are still mad good, especially with access to Ridersbane and Blessed Lance, and later Banish as Falcon Knights.
Back in Gaiden, the Falcon Knight's Banish skill was effective damage against all Terrors, but it's been nerfed to a flat 10 damage bonus in Echoes. It's still pretty much enough to trivialize most Terrors you'll encounter on Celica side with the Blessed Lance, though.