I have an Idea! one of you please try it and tell me if it works! I call it:
The Zombie Train of Death!
so trap about 20 zombies in a small, 4x4x3 dark room, with a small 1x1x1 doorway(no door). then, build a bunch of tracks around your world, starting with a track in the doorway designed so that the track faces the wall of the "house" and so that zombies can get trapped there. then put a block(preferably cobblestone) at the other end of the track. (make sure to put power tracks every 5 spaces, and have the block at the end of the track be no more than 2 blocks away from a power track.) this is what it should look like from above

I think you know what the letters are) (T=track, Z/z=zombie, H=solid block, P=power track)
HHHHHH
HZZZZzH
HZZZZzH
HZZZZzH
HTZZZZzH
THHHHH
T
TPTTTTTPTTTTTPTTTTTPTTTTTPTTTTTPTTTTTPTTTTTPTTTTTPTTTTTPTTTTTPTTTTTPTTTTTPTTTTTPTTTTTPTTTTTPTTH
Put two glass blocks above the second track away from the hut. the zombies should be able to walk onto the first track, but not the second. they should, however, be able to go through the glass blocks when they're in a minecart. Ah, yes, the minecart! place a minecart on the fourth track away from the house and push it towards the house. a zombie should be forced into the minecart if it is standing on the track. Make sure that the straight track is out in the sun, so you can just sit AFK and watch the zombies go by, burning. the way the track is designed it should be so that the zombie in the minecart will burn to death by the time the cart reaches the zombie house again, making it so that the zombies get pushed into the empty minecart when it reaches the home again. If you want to, you can add more zombies to the house for better results and longer zombie killing. Enjoy and please tell me if it works!