There seems to be some speculation over the exact criteria for this achievement, however that could be due to the possibility that this achievement could be glitched.
Some people say this achievement is unlocked by unlocking all weapons on the top row of the primary weapons of each class except archer, however this didn't unlock it for me.
Other people say you need to get 100 kills with every primary weapon type except the ones in the archer class, however I unlocked the achievement before fitting this criteria. I personally had 100 kills in all weapon types except for the knights heavy blunt weapons, this achievement unlocked on my first kill with a heavy blunt weapon, therefore I think it glitched.
The method to make sure you get this achievement is to get 100 kills with every primary weapon except the archer class weapons and you will get this achievement somewhere on your journey towards this criteria.
If you have any more information comment below and I will add it in here.