Achievement may glitch sometimesRemember that this achievement unlocks when the fight is over, not during the fight. Sometimes it unlocks right when you see him running away, but sometimes it unlocks during the following dialogue. However, if you are sure that you've killed 50+ automatons during the boss fight but for whatever reason your achievement didn't unlock, that's because it may have glitched. From what I've seen, it happens rarely when you unlock the level 20 achievement during the fight.
Anyway, if that happens to you, simply
don't save after the dialogues following the fight.
Instead,
clear your console cache and do the fight again and this time you should unlock the achievement properly.
To clear your cache:X360:
-System Settings
-Storage
-Move the cursor on Hard Drive and press
-Clear System Cache
XOne:
-Press & Hold the console power button for 10s until it shuts down completely
or
-Power down the console, unplug the power cable and plug it back
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I tried to get this achievement 4 times (and for one of the try I killed robots for 20 minutes) but it wouldn't unlock until I cleared the system cache. I was playing on Xbox One but people have had this issue on 360 and apparently it can affect other achievements too.