don't be afraid to challenge your PC's, especially at what, 12th level?
Remember, similarly leveled opponents are proffessionals. They are good at what they do, and they use all the same dirty little tricks. They are going to try to engage mages in melee, or disrupt thier spells any way possible. They will pick of fighters with magic. They WILL dispel, it is very useful, so much so that it would be a common tactic amongst any higher level group.
I mean, if that cleric over there seemed a lot tougher than he looked and was cutting through melee like crazy, wouldn't you dispel the bastard? Area dispels would be common, target ones would not be uncommon. I mean maybe they notice someone improved invisible, or they know of the party before hand and have figured out how to counter them(12th level characters should have a pretty decent reputation afterall).
Combined arms tactics. Many groups, races, etc. have lots of different tactics. And they will cooperate to use them (unless they are new recruits). They will come up with some rather strange, unorthodox tactics. The NPC's aren't brainless.
Mainly, Variety! Switch up encounter types. Magic based ones, melee based, suprise type (do something unusual, or have somebody hiding, or an ambush, something) Multiple encounters (magic users are good for 2-3 encounters before they run out of firepower). Or combinations. Mix in Politics even!
I've gleened tons of ways to control players powers and make them diversify from reading Pirate cats storyhour. And in his most of them cast buff spells on themselves and others that last most of the day, EVERY morning regardless of expected action or not. And he challenges them with lower level encounters.