An ode to Rube Goldberg
20 chain reaction explosive kills
At the beginning of the second Mission you will be able to spot an explosive barrel. It’s a little hard to see but it will be marked if you look through your binoculars. You get a chain reaction kill towards the achievement everytime.
Make a savegame (progress is not lost after loading) and shoot it, then load your save an repeat until you have your 20 kills. Since this spot is right at the start of the mission it’s a pretty fast way to get this one done.