To unlock the Arrow to the Knee trophy in The Division 2, you will first need to gain access to Specialisations. These become available once you hit level 30 and complete The Capitol Building stronghold mission.
Once unlocked, you can find the Specializations vendor on a table next to the Quartermaster at the White House. You can choose between the following options:
Demolitions: Grenade Launcher
Sharpshooter: .50 Cal Sniper Rifle
Survivalist: Crossbow
You can switch to these weapons by holding
.
Choose the Survivalist specialisation, and simply shoot 10 enemies in the leg with it.