Is Soulbreaker (weapon enhancement) + Hunter's Mercy (spell) a legal combo for negative level-age?
It's tricky, as Soulbreaker is a melee-only weapon enhancement and Hunter's Mercy applies only to bow attacks. You would need to find a bow that can be used in melee. SoX's Yuan-Ti Serpent Bow works, for instance. (But only in melee, given the details - melee and ranged are enhanced separately.)
For similar reasons, the whole concept of
Manifesting Lucky arrows fails.
Manifesting Lucky is a melee-only enhancement, and you only get the ammunition x1/50 reduction when using arrows as, well, ammunition (ie, with ranged enhancements).