If even the Tau consider the Tyranids something not to be reasoned with, I'm sure the Eldar have some sort of contingency plan for hunting them down as needed. Though admittedly most the situations I can think of in that case is 'direct lesser race into line of fire and hopefully they will figure it out in their own filthy mon'keigh way.' Maybe stuff relating to Harlequin skins or corsair looks? Idunno, will need more exploration. My original idea was trinkets mixed with wraithbone and tyranid chitin, but surely something can be agreed on.