I finally found the hardest of hard counters to deal with people who bring deathstars.

from the daemon codex. The Masque. (basically an herald of slaanesh that can't join other units)

At the beginning of her shooting phase she gets to pick 1 unengaged non-vehicle unit within 12". that unit can only Move,Run, charge,fall back d3" and suffers -5 WS. No roll to hit or save involved in the ability.

All on a model that can deep strike.

She is like the culexus assassin of deathstars! for 75 points.

She also has a -5 ballistic skill ability for shooty deathstars.