I believe this guy is using a wallhack and an aim assist. Here are the reasons;

@0:08

@0:29 ( straight line up, strongly suggest aim assist there)

@0:57 turns right where the guy is behind that container. then player pass by.

@1:38 Goes down sight already spot on LA dice camo player.

@1:53 ( Obvious WALLHACK ) shoot through fountain like if theres nothing there

@2:48 ( he takes a first look but player not showing in flir sight)

@2:52 Player was there , his sight switched right to him and shot  him.

Snap @3:37 / @3:39 / 3:41 / 3:43 (obvious)

@3:45 went down sight right on target ( WALLHACK)

@4:09 Goes right on target but miss / He takes cover to load bullet and keep right on target @4:11

in review