The old thing where you could get a music disk if you make a skelly shoot a creeper still works in FTB
With mods you can easily make a farm using this mechanic.
I have a setup that I built a while ago where I have an EnderIO powered spawner spawn creepers in a 9x9 room, then MFR conveyors funnel them and cause them to fall into a 1x3 room and survive with about half a heart left. There is an opening on each end of the room that permits line of sight but doesn't let me or mobs through. On one end is where I stand, and on the other end is a name tagged skeleton (which I appropriately named 'DJ'). The name tag means that the skeleton never despawns. I stand in the skeleton's line of sight so that he starts shooting, but since the creepers are in the line of fire, the arrow will hit and kill one of the creepers (if there are no creepers, I step aside so that I avoid being hit). A vacuum chest collects the drops.
The only thing that really stops you doing this with vanilla only is the lack of creeper spawners. If you did cheat in a vanilla creeper spawner, you could probably do it with vanilla mechanics only (use flowing water instead of MFR conveyors). Although to stay within range for a vanilla spawner to spawn mobs, you'd have to shorten the fall a bit, so I would probably give the skeleton a bow with a high level of Power, and perhaps stand further away, so that the skeleton would still be able to one-shot the creepers. Vanilla hoppers could be used instead of a vacuum chest.