The required amount of kills for the hunter bot diamond badge is 100.
This seems daunting due to it only targeting elites and specials, which aren't the most common of spawn however there are a few tricks.
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Run the Hunter bot at the start of the match to avoid having to craft it or wait for a drop. They're unlocked at level 42
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Run the Picky Eater augment on Vulture Aid, this allows for your currently equipped lethal and tacticals to spawn more often than decreasing the chances of random equipment drops.
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Weaken the Bear/Doppleghast, the hunter bot doesn't 1 shot these enemies, typically less than quarter health should be enough. I tended to make the 1 shot.
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Extremely important to note that zombie kills count, this can be achieved via 2 methods.
A) Having zombies near the Bear/Doppleghasts when it launches at them
B) Whilst Insta-Kill is active throwing it into a crowd of zombies can kill around 3, although not a lot at first paired with the Picky Eater augment you'll be getting Hunter Bots back in no time.
[The Insta-Kill drop can be shot from the Silo on Vandorn Farm even on the survival map]
It took me 2 matches, once I realised zombie kills by just throwing a drone into them with insta-kill active, actually counted to achieve this mastery badge.