This must be done in a Gang Wars mode and is suggested that you have a full 8-player boosting lobby and a Sniper Rifle loadout. Be aware that the observation platform is not the 2-story building immediately to the left of the yacht. You can find the observation deck to the south-west of the yacht, past some metal catwalks. It is the single-story, white building with large glass windows.
Have your partner(s) on the enemy team stand on the front of the yacht while you position yourself inside the observation platform. Melee the windows and follow up with a direct headshot from your Sniper Rifle. The achievement will pop as soon as you kill one of the enemy players on the yacht.