Just had a mini heart attack. I will killed with all of that, and had to make a mad dash in the middle of the night for it. Luckily, I got it and am back at my base.
Randomly mining. Whenever I got a skelly and a creeper next to each other, I would go up on a ledge, get them onto the same block as each other, and have the skelly try to hit me, but hit the creeper instead.
You can make creeper traps and then get skellies to fire at them. If the creeper is killed by skelly arrow, it drops a disc. Every time.
Wow, that's lucky. Sometimes when I look in on my grinder from an observation deck, skellies kill creepers, which drop disks.
11 used to be unobtainable, but then it was added to the bunch along with Wait (which used to be an unimplemented track) in the 1.4.3 release.