I think OP is talking about the chase mission, where you have to run after Hickey and tackle him but not kill him. I believe one of the requirements is "Do not tackle/shove anyone." This one was actually quite the pain in the ass the first time I did it but I honestly didn't have any trouble tackling him. I just remember following him left into the alley, then when he turned right, I went straight. After some running, I came up directly in front of him and tackled (holding R1 and pressing circle). If that's not working for you, I also remember watching a video where Connor climbs up the side of a building slightly and tackles Hickey from above. Obviously, you'll have to find a way to come up in front of Hickey like I did. I hope that helps.