We were trying to get that achiv 2 times recently and despite of not even touching any of 4 towers it didn't give us any achiv, even partial. Does anyone know what might be causing it? Been doing it on other characters not so long ago and it worked perfectly fine, but not anymore.
The only things I can think of is that we left few small towers up or because we killed boss too fast and not all defensive systems activated, but that's only a guess. Anyone got any idea?