Not sure if the Death-Lords respawn. Never hung around long enough to find out. Chances are pretty good that you'll get at least 1 from this fight. I've gotten 2 each time so far though (there are 2 Death-Lords to fight).The Giant's Sword is a 2-handed, extended reach, Mook only sword.
Has to be Mook but can be Lord, Samurai, Valk, Ranger and I think one more profession can equip it.
Spence
I don't think they respawn (they're set-piece programmed encounters, not random ones). However, there are something like 4 Death Lords or their equivalents in the game, each of whom drop a Giant Sword. As to whether it's the "best" weapon for a fighter -- well, obviously your character has to be a Mook, and you have to be willing to use a two-handed weapon (yes, it's two handed!), giving up shield or dual weapon use. I really like the Giant Sword because of its extended range, and the fact that it's always there (unlike certain more random drops...). Still, stylistically, I personally wouldn't go creating an all-Mook party just so I could use them...