This achievement can be done in just about any Interval as Pointman. In interval 1 there will be groups of ATC soldiers on narrow catwalks. In interval 3 you fight a lot of psychos, they swarm you and give plenty of opportunity to slide kick a whole group of them. In interval 4 you'll come across a group of 6 huddled around a TV. When you can get enemies lined up; sprint at them, activate slow motion and use the slide kick and kill two enemies with the one attack. You will have to make it to Level 2 (20,000 EXP) to earn the Slide Kick attack.