The simplest, and possibly only way to get this achievement is to use the defense control crystals. Have one player designated to run around and activate them, or have several players, each assigned to one crystal. Since the crystals take a few seconds to activate the defense system, you should activate them the moment Ichoron disappears. There are five defense control crystals:
on the left wall, by Ichoron's pond
on the right wall, by Lavanthor's cell
on the wall, between Moragg's cell and the middle ramp, on the upper tier
on the wall, between Erekem's cell, and the middle ramp, on the upper tier
on the left wall, down the stairs, by Erekem's cell
Important detail: the tank should tank Ichoron in the pond. Since no add spawn from there, it will totally remove any bad luck you may have when tanking the boss in the middle or elsewhere, where adds may manage to sneak in before you can kill them.
Three crystals are required before Ichoron enrages and stops splitting. It may be possible to do this achievement by simply killing the globules very fast, but this would require a well-geared group who probably has defenseless already and therefore would have no reason to not use the defense crystals.
Dehydration
Ichoron in The Violet Hold on Heroic Difficulty without allowing any Ichor Globules to merge.
Edit
