To unlock this achievement you must reach 20xCombo. That means you have to kill 20 consecutive enemies (or shoot environmental objects) without letting the combo reset. Now... you can torture yourself trying to do it or you can just make it easier with "Waste Not" skill under Trapper skill tree. With this skill, while in concentration the combo multiplier will increase by 2 on each kill instead of 1, as a result you only have to kill 10 enemies instead of 20. Most of the times you won't be fast enough to kill 10 people in concentration before it runs out, but there is one opportunity to do it during Episode 5: Grey Wolf - Dances with Renegades. At a point you will hear Grey Wolf speaking and you will have something like "extended concentration". You will be in concentration mode for like 45 seconds and the enemies will keep coming down the hill to kill you. This is your chance to achieve your higher combo ever that will lead to a huge amount of experience paired with an achievement.