I've never had Bane work against Tiamat even after trying it for turns. The Bane Sword is good against large groups of enemies as when you use the sword to cast Bane, it often kills at least a few of them if not half of them. However, it rarely works against bosses, and it doesn't even work on all the monsters. The only super weapon I can think of is having your Knight on level 50, equipped with Masamune and having Haste and loads of Steel and Saber (from Giant Gloves) spells cast on him. I've done 3500+ damage on Chaos and 5000+ damage on a Goblin that way. Against Chaos, that's nearly an instant kill in WSC and PSX versions. It takes a few turns to make it do that much damage, though.