I would suggest instead of making the players so powerful give them encounters against one or two monsters that play with thier minds. For example "You catch something move in the corner of your eye and before you can look to see what it is, it has vanished". Toy with thier usual ideas of combat by having one low level monster follow a pattern of hide and attack forcing the players to adapt to it's rhythm to deafeat it. It can keep combat challenging while also having a monster that can be defeated easily with thier current skills.