Most Frustrating Enemy
I've been playing through some of the games on my back log (Half-Life 2 and early Mario games) and have come across two common enemies I love to hate.
Half-Life 2: Poison Headcrab
This little guy is like the regular head crab that's there since the beginning but takes your health down to 1.
Mario: Hammer Bros.
I find that if I come across these guys when there are two ledges to deal with, its fine. Just get below them and bump them. But if they are on the ground, it's very hard to deal with them, especially in The Lost Levels.