My group is playing Abomination Vaults, and on the second level there is a room with an undead Minotaur Skeleton Warrior. The book specifically states that it never leaves the room it's in.
My group quickly figures this out and would simply have everybody ready an attack, one PC would open the door, triggering everybody else's reaction, and would then close it again.
After doing this about 10 times they finally took down the skeleton despite it having resistance to most of their attacks, with none of them taking any damage.
This is a somewhat inexperienced group that doesn't usually do strategy things, so I was happy to see them plan and execute.
Anyway what's the weirdest way your group beat a supposedly challenging enemy?
I think it depends on how experienced the players are. Players without a lot of experience get a lot out of feeling like they outsmarted the game, and it can lead them toward seeing the wider possibilities outside of just hitting the enemy really, really hard.
But definitely don't let them do the same trick in multiple encounters.