A easy way to do this achievement on any difficulty is to do it in the very last quest of the Bloody Baron questline.
You will be attacked by a major monster (did not specified which one to avoid spoiling it) and a few drowners + 1 water hag. What makes it easier to do it here is that most of the monsters will fight the people that are also involved in the quest (Bloody Baron's squad and /or Church of the Eternal Fire - i don't know if it depends on what your choices are), so just keep close to the fight and last hit any low hp enemy without using quen sign or getting hit.