A common failing of comic books and action movies where the first time a creature is introduced, it is extremely difficult to defeat. It is often presented as undefeatable except by extraordinary effort and/or luck. However, as the story progresses, more and more creatures of that type are included in the plot, and the more there are, the easier they become to kill.

which bad guy don't you think this has happened to yet? Besides Doom obviously.

I was thinking less about individual bad guys and more about groups. For example, in the beginning of a 3 issue story arc, the X-Men encounter a single Sentinel. It informs them that it is a new model and it has ways to counteract all of their powers. Working together, they barely survive the battle by knocking it into a nearby volcano. But by half-way through the second issue, the X-Men are tearing through a Sentinel base, and Cyclops is singlehandedly blasting off their heads three at a time.