Too good, in my opinion.
I wouldn't scale the damage with level - I would just make it a flat 1d4 or 1d6 damage (I'm leaning towards 1d4), and the attack roll is made with the caster's BAB as normal (no additional bonus - the write up you have makes them as good with this as an actual fighter with a longbow).
As this spell will almost always be used by low-level casters, the "If the caster takes a full-round action to make a single attack, they gain a +2 bonus to damage" rule means that it will usually be used in this manner, since the average caster will have only one attack per attack action anyways, so if they don't have to move, they can stand still for the +2 damage... Maybe if it was a situation that they could cash in iterative attacks for +2 damage, then I could see it working better.
I would also indicate what kind of damage it deals.