UPDATE:
I’ve figured out what was wrong with the achievement after messing around a few hours with a friend online, it turns out the ‘Capture Master Slayer’ and ‘Servant Slayer’ trophies are swapped! We unlocked the servant trophy with no servants at all in the match only capture units and vice verse on the next match, it seems the amount needed is still correct just the opposite of what needs to be killed to unlock it *(so if you kill 20 servants, you’ll unlock ‘Capture Master Slayer’ and if you kill 100 capture units you’ll unlock ‘Servant Slayer’)*.
Just a heads up for everyone trying to get these I hope this helped.
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Hey guys don't know how active people are on this game but I've been trying for a few hours to get the Servant Slayer achievement (kill 20 servants with your master in a match) and I cannot for the life of me seem to get it. I've tried everything in exhibition mode, killing them in the lots of three to add up to 20, summoning lots of three 20 times and killing them etc. it just doesn't unlock.

For those of you that have it am I missing something? I have no other idea I can possibly think of to get it to work :(