Can I offer up the suggestion that Galyxx's Death Attack be calculated as normal for an Assassin, but treating his level in the Assassin class as being equal to his HD for the purposes of determining the save?  It'll be an extremely high save DC, but it's also an ability that takes three rounds of setup before he can use it, and is limited to situations where he could sneak attack normally, so it's not likely to see much actual use in normal combat.