Wilhelm's Nightmare Achievement in Homefront
Knock 10 enemies off of the scaffolding during the helicopter fly-in in Chapter 7: Golden Gate
How to unlock Wilhelm's Nightmare
In the very beginning of the level where you are being flown towards the bridge, you take control of your player. Use the grenade launcher to fire at the enemies and be sure to fire a little behind them so it will knock them off the platform. They must be knocked completely off the platform for it to count.