The net result of this is that you can gain a strong advantage in this fight by using archery or hiding (especially both together), and you will be at your weakest if you are an unconcealed mage. Melee warriors are more or less in the middle here, though if you have a hard time discerning Sigdis from his copies as you run from one platform to another you will be easily worn down by the zombie's archery talents.
Overall, the cups-and-balls game that Sigdis makes of his battle is difficult- but surmountable. Unlike with his brother Mikrul, there is no 'absolute losing' strategy, since he doesn't ever heal himself.
Once you've dealt with the Gauldurson brother here, roll him for some valuables- you can take his bow (useful), his writ of sealing (interesting but useless) and his amulet fragment (the whole point), which grants +30 stamina when worn. Head through the passage behind his sarcophagus and you can find a small treasure room- in addition to a chest, you can collect some pearls for alchemy, a potion, a book for Two-Handed Weapon skill, and a soul gem.
Continue past the chest to climb a narrow passage full of mushrooms that ends with a lever- the lever grants you an opening to the initial cavern of the dungeon, and your way out.
In order to enter the third dungeon, the ruins of Saarthal, for this quest and reach the parts you need, you're going to have to start the quests for the College of Winterhold, which are covered in another series of articles.
As the ruins of Saarthal are absolutely vital to the Forbidden Legend quests and to the College of Winterhold quests, this portion of the guide will be linked to via 'Related Articles', so that it can be reached from both article series. It will then have the continuance of both the College of Winterhold quests and the Forbidden Legend quests listed as Related Articles from the final article in its own series.