Laser guided kamakazi attack drone!
That is one possibility. The US military is already using the Switchblade drone against enemy ground targets.
http://www.youtube.com/watch?v=-dgvBb5ke-E
A small kamikaze drone would likely be infrared guided by the hot exhaust of a larger drone and fly into its engine.
If the drone is flying higher up, the attack drone could be sent up on a balloon, or even as part of a model rocket.
A small drone could have a microwave transmitter to try and fry the other drone’s electronics. But explosives are the most cost effective. So rig the attack drone with some short tubes with electrically fired shotgun shells in them.