He is highly resistant to slashing and crushing damage from regular weapons. A workaround is to use pristine weapons crafted from rare harvests.
To keep Sartar's strong knockback from killing the attacker (group or otherwise), consider one of two possible defensive positions:
A tree stands not far from where Sartar spawns for his first attack. Have the main quester (and the tank, if they are not the main quester) stand with their back directly against the tree so that Sartar's knockback will knock them directly into the tree only a few inches, rather than half way down the stream or into other mobs.
Position any group members besides the main quester slightly up the slope next to the walls of The Temple of Cazic-Thule. Then, have the main quester hail Sartar and IMMEDIATELY run as fast as possible back to the group. If Sartar attacks before the quester arrives at the wall, there's a good chance they'll be punted directly into the group anyway with Sartar hot on their heels.