So with the new white dwarf, issue 454, there are additional rules for harlequins.
One is the shrieker cannon Cegorach's lament, which has a shrierker round that has AP-3, D3, and more importantly:
"each time a model is slain by the attack made with the wail profile...it's unit suffers D3 mortal wounds and until the end of the turn subtract 2 from the leadership of the models in the unit."
They key word is 'each'. So each model that is killed, RAW the unit takes d3 mortals and suffers -2 LD.
The kick is when you combine this with the Death Jester Pivotal role Harvester of Torment, where each hit counts as 3 when shooting units of 6 or more models.
That's great anyway, you can get 9 hits with the assault 3 shot of Cegorachs's Lament, however this gives you 3 hits with the wail profile...potentially 3 wounds, leading to 3D3 mortals, and -6 to LD.
So shooting a unit of 10 tactical, you'll likely get 1 hit, which changes to 3, 2 wounds, 2 kills (ap-3), then another 4 mortals on top, for 6 dead. The remaining tactical marines now have lost 6 models and have -4Ld, which with an average roll 10 (6+d6) on the now Ld 3, is basically an auto failure and wipes the squad.
Even if this gets FAQ'd to be -2Ld non stacking, it's still a super powerful combo.