I dont really remember if I died in DS mode, all I remember was that after a long fight, i died. which was really lame on my part. 
Try fafnir, cut that bastard in half, beat it up a bit, then when it goes all rockety or uses that spinning poison mist attack, go ds, you will surely die. Plus the fact that when he splits up, its almost certain that its weakened.
Anyways, my tip is, dont use over powered weapons as it will kill them too fast. use weaker weapons, less attack up and such. You'll notice when bosses are weakened as they have certain animations. The big birdies, specially the one that doesnt fly walks like its in pain, Hati and skoll both shake their heads. You'll also notice that the bosses will tend to change their attack patterns. For example, menia and fenia will tend to use their spin attacks more often. when their near death. Try fenia, she sucks balls, she'll start using that spin move often when she gets beaten up good.







