I was having a lot of trouble with this achievement and tried it multiple times using the video in the other solution. What ended up working for me on my Expert play-through was the following:
Level: Surrender
Area: End of the bridge/Start of ForestPath after Death-Eaters stop attacking.
After fighting the initial waves of wizards a short cutscene will play and you will see 4 Death-Eaters standing at the end of the bridge. With 2 standing stationary on each side of the road I simply fired a Confringo spell at the middle of the road killing all of them in 1 simple shot. This would be even easier on Normal mode as during this part the Death-Eaters don't attack and always seem to die in just 1 shot