Personally, I'd much prefer it if promotions could have the <UnitClassTargets>, <UnitCombatTargets>, <UnitClassDefenders>, and <UnitCombatDefenders> tags, and especially if similar <PromotionTargets>, and <PromotionDefenders> tags were added too. That way, you could could make Marksmen have spells that let them switch out different promotions to target different units, rather than only going for the weakest. The Netherblade could also target Heros, and the Godslayers Avatars. You could get rid of the bonus vs Avatar and hide the promotion so that it is nearly impossible to know what unit could slay your god (well, apart from the magic immunity and extra strength.)
Would it be too much to make it so that The Godslayer could slay Auric Ascended without even an actual battle if and when he casts Snowfall_greater within range of the unit bearing the weapon?