I -think- dragons are considered elemetentals for the purpose of elemental resistance. I don't think dragons are considered outsiders for the purpose of banishing attribute. Note, the dragon killing bow (Wrymslater) has Beast Slayer, not Banishing.
Anyway, the beast slayer attribute looks better than the new skill, since the latest affects pure animals only and not semi-animals.
It is a pity that there is no way to test all this (or is extremely difficult) since the battle log doesn't tell if an attack triggered any special attribute... You had to. E extremely accurate and see if any attack did deal any damage greater than expected. It is the only way to tell if the beast slayer attribute did trigger or not.