When fighting the Courier, kill the Fighter first because of his knockback, then the DPS. If you try to kill the Courier before his 2 bodyguards are dead he will just run off and despawn.
Like the Fang of Regulus, the Powerful Remnant of Aedraxis is poisonous and will kill you unless you head straight for Thedeor's Forge and destroy it (unless you can heal yourself).