Echoing Strike

Insight is a good and cheap option for Echoing Strike to level your character through nightmare and even hell. It has mediatation aura, 35% Faster Cast Rate, 200-260% Enhanced Damage and 1-6 To Critical Strike. Which is all very useful for Echoing Strike.

A level 48 Warlock can equip a Colossus Voulge Insight (non Eth) by means of level 15 Levitation Mastery and at least 2 points of STR every level up. You can easily level up your Mirror Blades to 20, put 1 point to Echoing Strike for the synergy, and hit 5K. You don't need anything else other than a Colossus Voulge Insight to achieve this kind of damage. Add some form of dual leech and you'll clear Nightmare in no time. This DPS can also extend deep into Hell difficulty as well.

Remember that as of the time posting this, Echoing Strike doesn't scale with attack speed and scales with FCR.

Note that Echoing Strike applies Deadly Strike, Critical Strike, and Levitation Mastery's crit chance. They cannot occur at the same time but it effectively all increases your chance to double damage. It is also note worthy that you can spread the blades wider by casting it closer to the Warlock. The closer the cast location the wider the spread of blades become. The further away, the more concentrated it becomes.

-Freezing Targets does not proc on Echoing Strike.
-On Striking effects will not proc on Echoing Strike.
-Elemental damage, Poison damage, and effects such as cold damage and chill effects will still work normally.

Echoing Strike in action.

Note that the "additional echoes created every fifth level" synergy from Mirrored Blades only applies to hard levels and not soft levels from gear. So if you put 1 hard level into Mirrored Blades and your gear brings the level up to 5, you won't get an additional echo for Echoing Strike. That also means the number of echos will be capped at 5 since you can only add a max of 20 hard levels to Mirrored Blades.