This achievement is relatively straight forward. All you need to do is start Junkenstein Endless and put the difficulty to Normal. Play the first 12 waves as you would any normal game, and then just make sure you take care of the bosses as quick as possible on the bonus waves.
The recommended characters for this encounter is (although not necessary):
The Survivor achievement solution should help for this achievement if you are struggling.
This graphic should also help with the last 4 bonus waves.AES0P
and I were able to complete this and actually got to finish Junkenstein Endless after bonus wave 16.
Good luck to everyone attempting this!