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.