Ahead of the Curve: Lei Shen

These "Ahead of the Curve" achievements are Blizzards solution to raiders feeling like their accomplishments were cheapened when a few patches later, players could go in and easily defeat bosses that once caused hours of frustration. The only solace raiders previously has was "Check my achievement date, I got that so far before everyone," while now these provide a small reward for those who managed to clear content when it was current.

So far we have four achievements, including this one:
Ahead of the Curve: Will of the Emperor
Ahead of the Curve: Grand Empress Shek'zeer
Ahead of the Curve: Sha of Fear

And it looks like Blizzard will continue this trend in 5.2 with Ahead of the Curve: Lei Shen

These achievements were added on top of the "Cutting Edge" achievements introduced at the start of the expansion that were for heroic mode kills.
Cutting Edge: Will of the Emperor
Cutting Edge: Grand Empress Shek'zeer
Cutting Edge: Sha of Fear

And again, Blizzard will continue with these achievements in 5.2 with Cutting Edge: Lei Shen

These achievements will be phased out over time, becoming unattainable when the new raid tier is released. Meaning that after 5.2 is released these achievements(Normal or Heroic) for Will of the Emperor, Grand Empress Shek'Zeer, and Sha of Fear will no longer be attainable.

Note that the titles are not linked to these achievements so if you didn't manage to get Delver of the Vaults or The Fearless, you can go back and get them later!

Yeah, there's reportedly no new raid in Patch 5.3, which probably means Blizzard are continuing the pattern of raids then a smaller patch:
Mists of Pandaria -ToES, HoF and MV raids (5.0.4)
Landfall-Arrival on Pandaria of the Alliance and Horde (5.1)
Discovery of the Throne of Thunder raid and Isle of Giants (5.2)
5.3 will not have a raid, as evidenced here:
http://us.battle.net/wow/en/forum/topic/7592242035#1
So there will be ample time to defeat Lei Shen over two patches.