So, this is a way to combine Harlequin and Dark Eldar allies to make a resilient, very hard hitting unit that can drop in and precisely assassinate a powerful character, then continue contributing the rest of the game.
-2 x Harlequin Shadowseers with PML(2), both using Telepathy Discipline; one has Mask of Secrets.
-DE Succubus with Archite Glaive, Armor of Misery, and Web Way Portal.
-Max unit of Grotesques, with Liquefier Guns as appropriate.
Entire unit squads together, then uses Web Way Portal to deep strike in turn 2+ with no scatter, putting both the Mask of Secrets and the Armor of Misery within 12″ of the intended target (let’s say it is a Tau Riptide, just for example). That means that the target already has -4 Ld, which means, on a Riptide with base Ld9, it is operating at Ld5. Next, both Shadowseers use Psychic Shriek (primaris power, so guaranteed) to target the Riptide… on average, each Psychic Shriek that gets through is inflicting about 5-6 wounds, so a total of about 11 wounds if they both go off, which in turn means about 7-8 unsaved wounds after the Riptide 5++, killing it and any drones in a single volley. If you really want to have some insurance, bring Neuron Disruptors on the Shadowseers and a Blast Pistol on the Succubus so you can dish out some more AP2 goodness in the Shooting Phase and finish off the target (if required).
The Grotesques make everybody majority T5 and provide some major tanking (FNP and multiple wounds), plus can deal with hordes with their Liquefier Guns and multiple close combat attacks. If any of the Shadowseers get Invisibility, then you can basically make the formation invincible on the turn it comes in (or even later turns), giving it time to continue to put out more damage in both shooting and close combat. Alternately, if you get The Shrouding power, then you can combine them with terrain for very strong cover saves.