http://67.23.190.71/forums/showthread.php?t=26031
I found this old post about the same topic. It appears, from the suggested working code, that the "from" variable needs to be $npc, simply listing the source of the damage as the script npc in question.
I will be trying this later today when I get the chance, although I'm still worried about the spell_id being 0.
Code:
$npc->Damage($npc, $h_dmg*0.1, 0, 1, true, -1, false);