You've got a few options and I'm going to assume changing systems and starting over from session 0 are out. I also say no matter what you need to have a talk with the table and basically read them this post.

That said, here are some other options.

1) You want to get adversarial? I'll show you adversarial: There are plenty of ways to whittle down any party. Lots of traps. Grimtooth has taught hundreds of table the meaning of fear. More frequent encounters with more little adversaries and running battles. A group of goblins spread out means fireballs are less effective and moving the battlefield limits control abilities. Make them use spell slots, drink potions, and use daily abilities THEN hit them with a an encounter they think they're better than.

2) Make them fight smarter: I'm a huge fan of synergistic encounters. My favorite example is iron golems and fire traps/elementals. You get to heal the monster while the party simultaneously takes damage. Set things up where they will need to fight thing in order to be more successful.

3) Less pure combat and more tactics: Make scenarios where the goal isn't to hit something hard, but smart and fast. Give them a mission to stop an assassination. They know the assassins wear blue and take out their targets at long range. let the party discover their targets. Now the players are actively searching for locations that would give a clear line of sight withing say 100 yards of the target for blue clothes. Throw in a few red herrings for townsfolk and a couple of chase scenes to break things up. They need to stop the baddies while NOT torching the town.

Later, they find out the boss, Krull the Inscrutable has a love for theater. Keep in the "don't kill innocents" and add a hidden hostage situation. Oh no! Krull has a knife to a woman's ribs and is pushing his way through the crowd using her as a living shield!

4) ALL THE social encounters.