Give Him the Stick
A little thing I discovered for this achievement.
Give Him the Stick 20
In Mission 7, in solo Campaign, take out both Hunters using only the Sticky Detonator.
When I played this through the first time I got this achievement on Heroic by only finishing off the hunters with the Sticky Detonator. Buy this I mean that I had already shot them up a lot and hit them with grenades before killing them with one sticky each.
There was also a Sticky Detonator in the office at the back of the room you find them in, far right side, that I used for this.
Hope this is useful.