A strategy for dealing with Swiftrunner is, if possible, to have all of the party members attacking Swiftrunner himself. Encounters with Swiftrunner are a key part of the quest and he is consequently invulnerable. If the first meeting results in a fight, once Swiftrunner's health is depleted the confrontation ends. The outcome is the same for the second confrontation. Lowering Swiftrunner's HP alone will end the confrontation.
See: Dragon Age: Origins