Build a 3x1 hallway in a netherfortress with both ends open, 15 or so blocks long.
the last 2 blocks put half slabs down. Lure a wither skelly into the 3 tall end of the hall and high tail it to the slabs.
The Wither Skeleton is a 3 block tall mob so will be stopped at the slab. Stair at him until the progress bar fills.