So I will preface this by saying I've been ruling that a creatures base HD (1 HD in the case of Humans) is what is affected by this spell ignoring levels if the target has them. Pretty hard on high level NPCs and PCs.  I was going to ask if  Levels = HD for purposes of spell (like in AD&D) when I looked at class descriptions again and realized that tables show number of hit die for each level.  So question answered I guess!  (insert head smack emoji)