So in English the term comes from back in the day when in games you could kill someone instantly in one shot, like sniping them. Then that term got adopted more widely when discussing fictional series to mean instant one shot KO from one move ... one-shotted etc.
So normally if you were to use this term it would imply the person was instantly KO'd. An example of being one shotted is when Kaido thunder baguad Luffy back on Part 1.
In the Killer fight, Zoro wasn't down instantly and in fact carried on fighting to defeat him.
The correct term to use would just be to say he was KO'd by Killer or something. Not 'one shotted'.