Stealthed gretchens in melee stance. Rangers can't detect without using their sensor, but gretchens can detect them. Gretchens only cost 50 requisition, they are dirt cheap. They can easily last against rangers in melee, which do 1 melee dps. They are so cheap that you can force the eldar back simply by sending gretchens, and moving the rest of your army up simultaneously. I've done this even in 3v3.