I think the trick is not having a certain "magic bullet" that's the only way to kill the monster, but rather being open to unorthodox wacko crap that desperate players come up with when their pointy sticks aren't working.That second line should be on a bronze plaque somewhere. Magic bullet monsters are okay in small quantities, but in general I think D&D is at its most fun when the DM doesn't have a specific endgame in mind. Set up a situation, drop in the PCs and react to the PCs. That's the most important part of being a DM. Someone else could be the rules guru at the table and advise you whenever something technical came up. Your main job behind the screen is to remain open to the infinite possibilities ahead of the party, collapsing the waveform only when necessary and with as little personal bias as possible.
It's the players' job to be clever, the GM's job is to give them something to be clever about and to inflict the ramifications of that cleverness in an even handed manner.
Back in the nineties I used to play Champions with a really cool bunch of people here in central Illinois. Whenever a GM's carefully constructed plotline went pearshaped they would break out this bit of wisdom: if a referee plans for X responses to the situation, the players will come up with at least X+1 approaches to the problem.
Nowadays I like to set X equal to zero and see what happens.