This ability is useful at first but once you start getting to the higher levels and can hit everything even when rolling a 2 its pretty pointless. Arcane Archers are still lacking a bit, depending on manyshot and even then you can get unlucky on slay living hits.
Simple change, why not make it roll a 20 when you use it? Will make the ability useful but far from game breaking when you can only use it every 30 seconds and things have so much hp in 18+ dungeons/raids.