I don't know how it turned out in the movie, but there are rules that Xanatos can use to make himself invulnerable. For instance, he could set his death well after it is feasible for him to die. Even if that wouldn't work, you can alter the time of death within 6 minutes & 40 seconds of writing it, therefore, he could use a computer to keep altering his own death. Also, if he's over 124, the Death Note will have no effect on him, which brings the time loop into question.
Then there are ass-random exceptions, like how B was born with the Shinigami Eyes.
Of course, it's also a rule that "All humans, without exception, eventually die." But that doesn't seem to be the case in Gargoyles, so you could probably suspend that one.
Speaking of Gargoyles rules, if you interfered with the death using sorcery, such as turning Xanatos to stone at 38 seconds, that may or may not negate the death.
I didn't see "L" coming, & you'd still have to guess "Lawliet."