Call of Duty 4 has a beast of a campaign on Veteran mode. It's probably the toughest difficulty I've played in a FPS (certainly the CoD series).
Be sure to hang around after the credits and you'll get to access a bonus mission. This mission is accessible on any difficulty, however you can only unlock the 10-point Mile High Club achievement if you beat the mission on Veteran. To be clear, you can unlock on any difficulty. You can even access it from the "Select Mission" option of the campaign menu. However, the 20-point achievement is only unlockable if you beat the last mission on Veteran. Spoilers ahead.
The bonus mission has you cutting your way into an airliner which is in flight. You drop in and you've got 60 seconds or less to secure the VIP hostage on the second level of the airplane. You'll have to fight your way through 20 to 30 enemies to reach the VIP.
Here are video tutorials that will help you unlock this achievement:
