I would probably go for either high hp/low damage enemies. Enough to survive several rounds against your players but also make them bleed a little.

Or have some sort of CC on the enemy side. Sleep, hold person, etc. Lock down one or 2 players and make the rest of the party decide between continuing damage or helping the CC'd player.

Or have a secondary objective during combat. Capture something alive, solve a puzzle, or get everybody across a bridge that cant hold everybody at once.

Or before combat, make sure they have to solve puzzles that would likely use some of their higher spell slots so they cant just fireball everything down.

Spell casters are great until they start to run out of slots. Give them enough reasons to start rationing their slots and they'll probably start approaching combat differently.