Last night i just finished a super-hero style 5e game played at level 13. We had a couple of neat character builds, such as a paladin/sorcerer ironman type, an elk totem barbarian 'speedster' using boots of speed, an unarmed unarmored paladin played as a cosmic 'captain marvel' type, and a sun soul monk supersaiyan-type.
What was most challenging for the group was cooperating IC, coming up with combat encounters was not too difficult. Challenging the party without the combats being a long, slow resource drain or a quick burn of weak enemies or a game of rocket-tag /was/ difficult. Very rarely did my heroes suffer any significant resource drain due to the short length of the adventure and being allowed 2 longs rests in adventures short duration (3 in game days) but they did face interesting challenges that they had to bypass within these encounters.
I included several lower CR creatures with legendary actions or multiple turns in combat, 'skill challenges' built into the combat such as needing the players to lure a villain through a night time chase to areas lit by floodlights to make then vulnerable to attack, or having them deactivate machines that reduced a villains immunity to damage (but reduced ability to harm then) to 3/4 cover, resistance, 1/2 cover, and then regular damage/no cover.
The monk was very adept at avoiding attacks/damage. the speedster could move VERY far in a turn and could move in then away from danger on top of having rage resistance and many hp. The ironman had high ac, ancient paladin magic resistances, massive save bonuses, and (at the end of the game) armor of invulnerability. Characters at this level are VERY hard to 'challenge' by means of comnat hits/damages/save effects, unless yiu pepper them with above-cr-appropriate encounters or use 'Problem-solving' encounters like i did. I played with.a group that is decent at optimization, so this was my experience and solution because not everyone in the party wanted to always kill everything or have huge long difficult fights all game long. Your mileage may vary.