What is the difference between damage with tank, and tank? It depends on build or what?

I have a problem with my achevement named "Fa"thra minions" where i have to kill 10 Fathra gangsters, and i have already killed more than 100 of them. Weird is that i got the second achevement where i have to kill 100 Fa'thra gangsters. Can you fix it? I can't get 100% of Hutta