I'm guessing indomitable is great. I've noticed I. The rare high level games DCs creep up around 20.
Even spellcasters with less than great saves are going to flunk that a lot.
Had a few "look at the pretty lights" scenario. Translation save at 15% chance or whatever to have your turn.
We are above 20 for most monsters that matter, but with Indomitable and Heroic Warrior the Fighter can make a 30 DC easily.
He also has Mage Slayer and has not failed a save that matters in a long time.