This is a fairly simple achievement. I got it in the first few fights after I met Solas.
A combination of Winter's Grasp (which freezes the opponent) into the Warrior's Mighty Blow (which has the impact detonator ability that gives you the combo achievement) will do the trick and that is how I got the achievement. But combinations happen very frequently if you have all classes in your party, so this achievement should be no problem.
I am unsure of other combo's for different classes as I have only played Two-Handed Warrior at the moment. So feel free to comment and I'll edit the solution where necessary