As for hunter's mark I'm not sure you can reapply it to the second horde breaker target, but it costs a BA so if you certainly wouldn't be able to reapply in the same turn you apply it (unless below 50% and wearing helmet of grit).
I see, this means that the Hunter's Mark must ideally be cast on the first turn. On the second turn, the target with the Hunter's Mark must be near another potential opponent who will get the Hunter's Mark off if I take out the first target.... Hmm, that is indeed a few too many conditions that need to be met. I think you are right. Colossus Slayer is better because at least you have the BA free, which is more versatile.
