My best recommendation is to use a character who is equipped with a heavy weapon with the Clear Out skill. Krzysztof and Wanda automatically come with a PIAT. The Clear Out skill takes 2 stamina points and must be used from the back two rows. When this skill is used it will do approximately 25-30 points of damage per target in a row of three. I would advise damaging targets in the row until they are under 25 points. Remember you can also use melee skills like Drag and Shove to move targets around. Some pistols also have skills which can push back targets. Once you have three weakened and in a row, use Clear Out and hope the RNG gods are kind and don't miss a target. If it does, you will have many opportunities while doing the multiple playthroughs this game requires to get all the achievements. If you are still having trouble, think about using XP on the accuracy trait. Alternatively you might get lucky with the grenade skill Destroy...although it chooses random targets.