There's something about zombies in Minecraft that if you merely drop them down 23 blocks for the fall damage, they take two punches to kill, rather than one punch like other mobs. (23 ½ is the same, and 24 blocks will simply kill them.) I’ve heard that this is due to zombies having “armor” (not visible armor, evidently) that has to be broken first (or perhaps it has something to do with the effects of rounding on armor damage reduction).
How can I automatically soften up the zombies a little bit more so that they only take one punch to kill, preferably in a way which ends in a drop of at least 11 blocks?
The mechanism must be fully automatic and accept zombies arriving through a water stream. It's OK if the occasional real-visible-armor-carrying zombie still takes extra hits.
What doesn't work:
Fall damage and suffocation damage seem not to be able to “remove the armor”: I have not found any combination of two falling stages, or 1 tick of suffocation followed by falling, which will result in a one-punch kill. (If you have found such a combination then I'd love to see an answer with a complete design to demonstrate it!)
There is the possibility of dropping mobs through lava, then water, which seems like it should work, but has the disadvantage that in order to prevent the mobs from swimming up in the liquids (thus dying to lava, or getting stuck in water), a simple tube would have to have, in order: a long drop, a lava block, (another long drop? not sure), a water block, and finally another long drop to the collection pad. This extreme height would make my underground zombie trap's plumbing poke up into my above-ground buildings.
Mobs spawn in this gigantic tower...
...and hit these fences. The water then funnels the items into the middle (which then connects to another stream flowing the lava you can see, to kill any mobs that fell through the middle).
Best Answer
I have invented a solution to this problem using a mechanism to apply one tick of lava damage. It occurred to me that a tripwire could be placed immediately above lava in order to remove it (using a dispenser) as soon as a mob hits it, thus preventing the mob from continuing to swim in it. Doing this for the water as well as the lava means that no acceleration is needed to make sure the mob will fall down, so the whole mechanism is only a little bit of extra height above a standard fall-damage trap.
Build details, from top to bottom:
The zombies are pushed by water into the drop pipe. I don't know how important the length of the upper section is.
The water and lava mechanisms are identical:
Each dispenser contains an empty bucket.
All pistons are sticky.
The tripwire above the fluid is the input to the left (in first image) piston monostable (rising edge detector), which causes the dispenser to remove the fluid into the bucket. The monostable ensures that the tripwire input will not interfere with the second fluid-restoring pulse.
Behind the dispenser is a comparator followed by 2 blocks of redstone wire; this detects the dispenser having a lava bucket (which counts as 1 stack) instead of an empty bucket (which counts as 1⁄16 stack), and triggers the right piston monostable to remove the fluid. In the event that the state of the dispenser gets reversed, this circuit will simply not generate a pulse, thus correcting the problem.
Below the water block is exactly 19 ½ blocks of empty space (counting the sign) down to the fall-damage landing platform.
Results: When the zombies hit the platform, a few still take 2 punches, but most of them take 1 punch to kill. I have successfully operated this mechanism in my survival world fed by two zombie spawners.
Glitches:
The amount of damage done is not completely reliable.
It is possible for the dispensers to get inverted (resulting in zombies falling on fire, without taking any lava damage, or from the wrong height). This latest revision with comparators can self-correct this problem, but it still may result in an occasional zombie with the wrong amount of health. (I have also seen a dispenser completely fail to respond to pulses, so I suspect there is a Minecraft bug involved.)
I know it can be hard to build mechanisms from screenshots, so if you would like any further explanation of other camera angles, feel free to ask me to add them.