I agree with the suggestions of assistance, barriers, and a backup plan if things don't go his way. Environmental issues can make the party change tactics, too; for example, a room full of tall stone columns makes it hard for the archer to get a clean shot, or for the wizard to get line of sight for damage spells. Spells like Reverse Gravity, Magnetic Field, and Antimagic Field can throw off their plan too, especially if it isn't obvious where these effects are or the party is unprepared for them.

I've also had good results with boss fights that challenge the players to be more creative with their action economy. It's easy to kill the big bad guy when you spend your full round nuking him down. But if the party has to balance the fight itself with tasks, challenges, or puzzles, it becomes much more interesting and difficult. I recently put my (low-level) party in a room that summoned elementals every round. There were levers throughout the room that when pulled, would destroy one type of elemental. They had to figure out how to spread out safely, which character to position at each lever, and who would save their round to fight non-elementals that could not be dealt with by pulling levers or heal anyone who got injured during the fight.

You can also give them simultaneous responsibilities they have to juggle during the fight. Do I fireball the bad guy or cast sleep on the henchmen who are about to kill civilians? Do I charge the boss or disarm the indestructible golem that's raging around the battlefield? The floor (and / or ceiling) is slowly crumbling away but I have the bad guy right where I want him, do I try to finish him off or run away before the environment kills me?

Lastly, give the henchmen something interesting or surprising beyond big damage rolls. Archers with improved initiative and Hushing or Sleep Arrows can really mess up the casters game plan. Guards use trip and disarm instead of straight damage rolls, and each one of them chugs and Enlarge potion as soon as the good guys show up. Tag on some completely unexpected powers or potions to really surprise them, too. I ran a session with experienced players once and they laughed in the face of approaching kobolds and prepared their traditional "wipe out all the wimpy enemies at once" tactics. The players gasped in shock when the first kobold used his firebreathing attack and the mage's fireball was completely ineffective. Maybe some demon or god or whatever gave your henchmen some extra powers or resistances that will throw a monkey wrench in the player's plan, or a counter to their go-to strategy.

-JonathanPDX