Add stuff to the scene that's not just damage. Stuff that splits their attention and requires some strategic thinking.

• Mooks are doing a ritual that will have bad results if it's completed in two rounds.
• Mooks flee to sound the alarm/get reinforcements
• innocent hostages are at risk
  • some of them are fake hostages and will attack if "rescued"
  • some of them are actually innocent, but are dominated/acting against their will
• any number of gimmicks that force people into smaller groups.
  • the boss is tangible only to one player at a time. Like that time in bg2 when I cast time stop and everything stopped except my wizard, and the demogorgon.
  • the boss is split into X parts that need to be killed at the same time , in separate rooms.

Also, playing Mage and Fate most recently it's been really refreshing not having any of these DND problems.