Oh yeah, right. What do you think, +4 bonus for favored enemy? That would give the weapon 2d6+6 extra hp of damage vs dragons compared to other critters, or +13hp on average. I guess we could bump to +6 favored enemy.
The original text just says "Undead dragon slayers use long swords (80% likely) or two-handed swords (20% likely). This weapon functions as a sword +2, dragon slayer in the slayer’s hands, and the slayer can attack three times in every two combat rounds." It doesn't seem to me that "attuning" is required. Sounds like some sort of hasting might be involved or something like Whirlwind Attack . Could be something we swap into the "menu."
Anyway, here we are, adding a proposal for the casters:
Favored Enemy (Dragon) (Ex): Undead dragon slayers have dragons as favored enemies with a bonus of +6, as per the ranger class ability.
Dragonbane Attacks (Su): Any weapon wielded by an undead dragon slayer, including natural weapons, gains a +1 enhancement bonus (does not stack) and the bane property against dragons. This is a supernatural ability of the undead dragon slayer and not of the weapon itself.
Anti-Dragon Casting (Ex): Undead dragon slayers gain the benefits of Spell Penetration and Greater Spell Focus for all spells cast against dragons. In addition, an undead dragon slayer may Maximize one spell per day at no cost if that spell's target is a dragon.