Team Play Achievement in Assassin's Creed Origins
Kill an enemy with a headshot while they are being harassed by Senu (The Hidden Ones)
How to unlock Team Play
In order to unlock this achievement, you need to have unlocked the Eagle Harass skill, which is in the left skill tree up near the top where it meets the middle tree. It costs one skill point, so if you don't have it already, make sure you use a skill point from leveling up to level 45 during this DLC on it.
Once you've got the skill, the rest is pretty straightforward: find an enemy, preferably a relatively alone one, switch to flying with Senu and then press when near the enemy to harass him. Now pull out a bow (predator is probably your best best) and shoot him in the head. You can additionally use the slow motion with the predator bow if you have that skill to make this achievement even easier. Once you get the headshot, the achievement will unlock.