TimeShift

Host a player match with custom presets. > Full time at start > very fast time regen > Insta kills > Instant respawn. On Rooftop excess MP1 with a score limit of 4.

Have your boosting buddy shoot to the side of you and then you throw a reserve time grenade in front of them and they will die. (a reverse time grenade is hold LB + B and then just LB to throw) Rinse and repeat another 2 times and it will pop in match.

Collect enough time to be able to throw a reverse time effect grenade. As an opponent starts to shoot you, throw the grenade at him/her and their own bullets will reverse back into them, killing them! This is much easier to do with a buddy, but still doable in ranked games.

To get this easily, and with as little hassle as possible, use these settings.

Find Your Boosting Partner

Player Match > Host > Custom Settings> Time To Full & Quick Refill > Instant Kills > Instant Respawn > No Warm Up > Rader Show All > Force Weapon And Starting Weapon To Rocket Launcher > On Rooftop Excess (MP1) > Any Score Limit Higher Than 3 (To Compensate For Errors )

Find Each Other On An Outside Part. Then Simply Get The Player Going For The Achievement To Throw Down Their Reverse Time Grenade (LB + B Then LB) In Front Of The Other Player By About 10 Steps. Other Player Fires Their Rocket Launcher At The Reverse Bubble And It Will Bounce Back At Them To Kill Them.
From Experience They Will Need To Aim So That When The Rocket Bounces Back, It Hits At Their Feet. If You Don't, More Often Than Not, It Will Pass Right Through. Weird!