Tyrian Protector can be a pain. As Fexghadi mentions, the Dunes of Despair mission bonus is notoriously annoying (you basically need to use a game "glitch", or effectively split your 6-person party in half).
Tyrian mission bonuses do not require you to complete the mission for credit.
True, but my point was more that you may need to split your party enough that you can keep the ghostie alive while you complete the bonus.
Easier if you just skip Goss at first, kill the enemy Ghostly Hero, start the countdown, then kill the Goss. It'll teleport the Ghostly Hero right before the bridge, where he's safe so you can take your entire party out to complete the bonus.