Hi there, fellow DMs!
I'm a fairly new DM (as in: I have around 20 sessions behind my back), and while my players seem to be enjoying the campaign, I've run into a bit of a problem.
Namely, that the three godsdamned paladins are trivializing most combat encounters.
They just leveled up to level 8, but even at level 7:
- Attack rolls against them? LOL, CR 7-9 enemies usually have +6-+8 to hit at most; they will miss the paladins (and the cleric) in plate armor + shield 60-75% of the time.
- Saving throw abilities and spells? Fuck me, aura of protection, everybody gets +2 or more to all their saves.
- Even if a spell slips through? Ancients paladin. Whoever came up with the Aura of Warding at WotC deserves a kick in the head. Everybody near the paladin takes half damage from every spell (quarter if they make the save) because balancing encounters is soooooooooooooo easy!
And that is just their passive abilities. There's of course the usual issue of smites (the three of them can easily deal 24d8 damage in one turn, that's 108 on average - and that's without accounting for crits or them stacking a smite spell on it too). Ranged enemies? LOL, orbital laser goes BZOT! (Moonbeam) Or they'll just leave them to the ranger, cleric, and the warlock. And if they still get banged up, they have 105 HP of dedicated healing between them (plus the cleric and the ranger).
Is there any way to make combat encounters challenging for this party besides trying to overwhelm them through action economy (it's a party of 6, so that would take a shitton of monsters and turn the combat into a slog), finding a way to force them into 6-8 encounters between long rests (wouldn't do anything about the passive abilities but it would at least curtail the smite-nukes), or turning the game into Dark Souls with every monster being a horrible damage sponge that can one-shot any player on a hit?
Because at this point I'm afraid that anything shy of a tarrasque would be a minor inconvenience at best instead of a challenge or a boss.
Have you considered an anti-magic field area of considerable size? That's hardly a good continual response but it may allow a more challenging encounter, especially if the field can be briefly turned off by characters expending spell slots into a single dangerous spot while being attacked. Since it prevents Divine smites and the bonuses. (Someone could 5e-argue that probably, since its pretty nonsensical with class abilities at times but I'd just overrule it.) You can even use it with Mindflayers or other psionic aberrations, since arguably their abilities can still work if you rule they can. (Psionics don't necessarily equal magic unless you set some other precedent.)
Other concepts are to add hazards and chaos causers to battlefields. Floating clouds of glittering fog that reflect spells to random other targets, negative-energy/undead quicksand bogs that leech endurance instead of doing damage (and whose saves are to avoid getting stuck not to avoid the drain), unsteady floors to drop out and separate parts of the party, or lair effects that randomly teleport characters back to earlier areas of a dungeon, forcing them to run back through to get back into the fight (don't overuse that one but if there are traps earlier its a great way to force them to ignore traps in their rush).
Also just tasks to do while being attacked. The paladins + cleric can defend well together but force them to be separated into different regions doing a task and you up the tension (even if you don't up the danger).
You can also try bringing forward older monsters that undercut benefits/items. Black Puddings, Rust Monsters (Or their papa Annihilators), or port over other monsters like Magerippers or Spellweavers from 3.5.
Traps can also be good, since they may be taking 1/4 damage on a save, but if the traps also cause inconveniences or force them through alternate, and slowly more damaging paths (like crawling through a stone brambled tunnel instead of taking the other tunnel because it caved in and almost crushed them), they can still build up and be valuable.
Also if they're incredibly proficient in combat, the enemies probably know that by this level. So you might have to start attacking them legally or socially, depending on the situation. Or just start having foes avoid them. Make them burn resources to set up engagements where foes can't run or attack them when they're on their back foot relaxing with assassins in the bathrooms, poisons in the bar bread, false accusers of horrible crimes on the streets and in court. (Depending on who their foes are of course.) You do that so that their character get paranoid. Start trading things like spell slots, the benefits of sleep, or close allies to try and defend themselves, so that they're weakened before they even touch something like a dungeon. If they party is just too invincible in combat as it is, don't just attack them there, let them know that existence is sortof a threat. (And as before, how much you use it is important an should be informed by session 0. You want them tense and excited, not miserable, and "the world as the DM's weapon" isn't necessarily the right way to approach it but its a nice tool to have in the chest. Hammer finds nail and all that.)