Runners are the enemies that try and slide-tackle you. In most cases you need to jump over the Runner initially. Then stop and go back to try to defeat them. Use any technique you like. There are more than 10 Runners to defeat, even if you only play each level once. Just defeat all the Runners you come across. Level 2-2 is good to complete multiple attempts as it has three runners. This achievement has been reported to be glitchy by some and does not unlock after defeating 10 runners and requires more defeats or a complete game uninstall and reattempt to unlock.