There was one level in
Tribes: Vengeance where a few satellite dishes had to be protected. I almost couldn't complete this level, even when I tried with a trainer!
Out of about ~350 completed PC games in my list, I found the following a bit too hard for my taste:
Aliens versus Predator: The first one.
Chrome
Command & Conquer: Renegade: Especially the last two missions.
El Matador
Enter the Matrix
The Great Escape: The train level.
The Hulk: Hard bosses.
IGI-2: Covert Strike
Iron Storm
Legacy of Kain: Soul Reaver: The first one.
Little Big Adventure 1 and
2
The Lord of the Rings: The Return of the King
Mafia
Manhunt
Medal of Honor: Allied Assault: Spearhead
Medal of Honor: Allied Assault: Breakthrough
Neverwinter Nights: Shadows of Undrentide
The Nomad Soul /
Omikron: Some of the FPS sections.
Nox: The tower of the wizards and the last third was hard.
Psychonauts: The meat circus in the end.
Severance: Blade of Darkness
Shadow Ops: Red Mercury
The Thing: Some levels were easy, others were hard.
Tomb Raider: Chronicles: Especially the last two levels.
Urban Chaos
Vampire: The Masquerade - Redemption
Vietcong: Fist Alpha: Unforgiving stealth.
The Wheel of Time