Oh, hey, I thought of something. There are two hard parts in Half-Life 2! The first is the final turret section of Nova Prospekt, though there is a trick to it that can make it pretty damn easy.
The other is the bit where you have to protect Alyx while she disables a shield in the middle of an enormous courtyard. There's very little cover in the courtyard, but the alleyways from which the Combine come has a fair bit - but of course, they're coming from all different directions, so you can't take advantage of that. Pain in the ass, that fight was.
But... not really "too hard", I guess. Unless you did the Nova Prospekt thing wrong. THAT was painful.