Difficulty balancing encounters
They're being creative because they want to be powerful. They want that "wow that's clever and highly effective, you're so smart here's a one-shot" moment. So, let them. Balance be damned. Let them wipe out entire encounters if they're clever enough. Or, throw in a fluffer enemy or two that can either get "one-shot" at any moment or be a nuisance for the entire encounter
Edit: If you like friendly competition (and you should probably check with your group too) you can turn combat encounters into puzzles where you're basically trying to stump each other on how to handle a situation. Try to think tactics instead of numbers
See my other response :)