The main reason why bosses are slow for Frost sorcs is that bosses turn off most of our damage buckets through immunity. However when you stagger the boss they are essentially frozen and chilled. So if you have the damage buckets you can usually nuke them within a stagger period. So here is the trick. Just spam frost bolt with the frost bolt enchant which adds chill %. Cold front (more chill while using a barrier) has even more chill and the icefall paragon board also has some chill %. When the boss goes to stagger switch to ice shard and nuke him at full mana.
You may think you are losing damage points by going chill, but remember chill turns into frozen, and with the right aspect frozen turns to vulnerable, so you are still adding dps for non boss fights.
At least for me 61 NM15, I can kill a boss much faster by dropping my damage to use ice bolt and push him into the vulnerable state than simply trying to dps him raw through ice shards. Boss is dead in like 2 seconds. However if you can sustain more dps than stagger progress just dps.
You will likely need the avalanche passive to sustain through the entire stagger period.
