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.