Assuming he's straight paladin, and we'll go ahead and assume that he's got a 12+ dex? That fair? Somebody mentioned rolled stats, I feel that's probably right. So he's got +5 or so dex saves from aura? Can you contrive a Rakshasa into the story at all? I envision an encounter something like this:
Preferably throw this at them when they are traveling somewhere, the Rakshasa and the other devils are out in the wilderness, or in a hidden room, or down a hard to spot passage in a cavern, or wherever is appropriate, and they are trying to summon a creature of some sort, maybe a Balor (they won't succeed, they're just trying to), and the Rakshasa is in the center of some eldritch symbol carved into the ground, decorated with fiendish imagery (humanoid fat candles with fractured and still slick with blood humanoid skulls to hold them, unicursal hexagrams, bizarre sigils, gore, salt circles, altars, ritual daggers wielded like wands, etc.) and the Barbed Devils focusing on the Rakshasa around the circle, with the bearded devils forming a larger circle around them all, each on a knee, maybe spread like a N/NE/E type compass, chanting something in infernal when the players stumble upon them, they notice the party (highest passive perception is 18 on the barbed devils, for non-stealthy characters that's a pretty good DC at almost every level, but still beatable, let them have the opportunity to ambush if they roll good, this is a hard fight
) and break their ritual and start attacking (the ritual could have just started, and if the party or the scout gets the drop on them then let them see them still setting up the ritual, or finishing getting into position).
1x Rakshasa (CR 13, 10,000 xp)
3x Barbed Devil (CR 5, 5,400 xp)
8x Bearded Devil (CR 3, 5,600 xp)
----------------------------21,000 xp---
Adjusted Encounter XP 63,000 (beyond deadly encounter even at 20th according to the table, but the barbed and bearded devils are very low CR compared to the Rakshasa, so I'm imagining they're going to get taken out due to AOE, but depending on party setup this may be an easy fight, or it may be a very, very hard fight)
If the Paladin is leading the charge (which he should be) then have the Rakshasa make himself invisible at the first opportunity while the other monsters all group up on the paladin (there's 330 HP between the barbed devils and 416 between the bearded devils, AOE is going to be the party's friend in this fight, and if they're all stacked on the paladin, the shining beacon of all that is good in the world, he's going to be in a world of hurt real quick). The bearded devils have glaives so they can make room for the barbed devils to get in melee as well (all the creatures have multi-attack, but the barbed devils get three, so let them get into melee, the bearded devils that can't be adjacent can still attack the paladin, and then use their beard to attack the allies, so you're still able to do things to the party) and then just let multi-attack do it's job.
The devils have zero incentive to let the paladin roam around and even less incentive to let the party stand in his aura (hence swarming him with the couple outliers who can maneuver around him with impunity since they're out of melee with the paladin, attack the paladin and his allies wherever they are around the swarm) to bolster their saves. You're looking at 10 attacks from 5 of the bearded devils adjacent to the paladin, and 9 attacks from the 3 barbed devils adjacent, with 3 more glaive attacks from the edge of the swarm, per round, against the paladin (24 attacks per round vs the paladin means you will win this war of attrition without the party AOEing).
All the while you still have Mr. invisible Rakshasa roaming about around the rest of the party. Illusionist bend, but that's ok, limited magical immunity makes your spellcasters either have to drop their big spells to hurt him (immune to 6th and lower), or rely on melee to kill him, and with your paladin firmly locked into combat with a horde of devils, the rest of the group is pretty well hosed, especially when you have the rakshasa dominate one of the other melee characters, or the party healer, but leave the AOE alone if they've figured out to blast down the monsters with area spells, which will also hurt the paladin barring a couple options available being in play, which is why the Rakshasa would leave them alone because the whole meta-game point of the swarm is to lock the paladin in place, give him targets to attack, and let the party use its own resources to take the paladin down a notch.
The goal isn't to kill the party, invisibility and dominate person are both concentration spells, so he can't exactly be invisible and take control of a PC (which he can spend his action doing if you want to take control of your AOE caster and burn off his slots for him
), which leaves him vulnerable. Also, bear in mind, these creatures, when they "die" they just go back to the nine hells, so they will absolutely fight to the death without hesitation. Which enables you to accomplish your actual goal; remind the paladin that he's not invulnerable. Good saves? Check. Good AC? Check. Solid HP? Check. Good melee output? Check. Invincible? Not. At. All. The Rakshasa harasses the rest of the party while the paladin is locked down in melee. Let's say he blows his smite wad, and is dropping 52 HP bearded devils in a round. You have 3 extra to keep him in melee with, and unless he spends his action to disengage, he's taking about 8 at-ops if he gets the opportunity to move (which he shouldn't thanks to being completely surrounded), giving you a minimum of 3 rounds of freedom with the "Haha you can't hit me with magic unless it's 7th level or higher, haha I'm evil" Rakshasa to harass the crap out of the party. If dominate fails (DC 18 for Rakshasa saves, pretty solid DC there) you still have charm person x3, and two greatsword-damage claw attacks to let the rest of the party feel a bit o pain as well.
I understand the feeling when a PC has a super high AC and defenses, and the monsters appear to just be wasting their attacks on the tank, so you want to just have them all kind of wander off to attack different party members because "why waste their actions?" But at the same time doesn't it make sense that the bulk of the melee would try and take out the tank first? I mean, going in they know he's going to be hard to hit already, so it's not like they were expecting to succeed (nobody in plate armor is easy to hit, and the more ornate the less likely to hit you are since magical armor is usually ornate) right out of the gate at taking him down.
I wouldn't waste time taking the help action with these monsters, you're getting 24 attacks per round at the tank from them alone, with an additional 5 potentially against other party members who get near the swarm, that's a solid attack round for team monster, the help action denies you a minimum two attacks per creature who helps, whereas if you just attacked you'll get more opportunities to hit. Help doesn't really "help" in this combat situation since the big bad isn't anywhere near the paladin (and also can fly and use a wand or staff to rain damage from above) and is there basically to distract the rest of the party while the paladin gets swarmed on for a while (but he at no point feels useless since he is at least able to attack and damage the creatures swarming him).
If you stick to the swarm just straight attacking, round after round, and do little things to encourage the party to AOE them all (maybe there's a stream or broken water barrels or a fissure leaking water, and suddenly that line lightning bolt turns into a blast within the water) including the paladin (unless they can shape the spells) then real fast the paladin is going to feel like they're threatened. Even with a +5/+6 to hit on the lower CR monsters, those hits are likely to be crits, which means you're looking at 2d6/4d6/2d8/2d10 +2/3, which is no laughing matter when you do hit. I'm assuming the paladin is rocking ~24 AC? So on the bearded devils you're looking at 19-20 as hits, and the barbed devils 18-20 are hits. Sounds hard to do until you remember that you get freaking 24 attacks on the paladin in a round. Suddenly those high roll requirements don't look so high. Especially if you have 8 or so d20 to roll, to let you speed up the round (8 d20 would let you finish the monsters' round in 3 rolls, barring targets for the outlier beard attacks).
This is not a fight to go full bore on, you're not looking for a TPK, you're looking to remind the paladin that he's vulnerable even with all of his mighty defenses, and to show the party that just relying on the paladin to be able to tank the world for them is not an effective tactic. Yes, this encounter is stupid high CR (somewhere above 20, between 24 and 25) encounter, but none of the creatures are particularly offensively powerful (the Rakshasa has only a few offensive spells, but none that deal damage) but they're solid on defense (HP, resistances, etc.), so I, personally, would feel comfortable throwing this encounter at a level 11 or higher party (9 if we rolled stats, 7 if it were a party of well optimized characters, but the tactics would be different, no swarming basically), especially if I did things to encourage AOE casting like a pool of standing water that conducts electricity, or freezes from cold magic.
Of course, things could quickly go south and you could wipe the party with this encounter if the paladin goes down too quickly (a run of lucky rolls on a single round could spell doom for the paladin, especially on tail and glaive crits, 4d6+3 and 2d10+3 sounds fun lol), and then you have a bunch of fire immune creatures turning on a party that no longer has a tank, so if you're not comfortable throwing what are technically epic level challenges at your party (they're at least 6th level right?), then by all means, don't, but worst case scenario is that the devils take them somewhere and are going to sacrifice them or sell them as slaves, or torture them for fun. An escape, should things go south, can be pretty easily contrived. If I were a devil, I would TOTALLY revive someone from dead just to torture them before spending all of eternity being tortured in hell, but that's me, and I like horror movies.
You can repeat this tactic with any monster formation that is similarly constructed. You want low CR creatures with multi-attack (gladiators are good examples if you want generic humanoid creatures to use), the to hit really isn't all that important, number of attacks is. Just swarm the crap out of the paladin, and don't stop. There's no actual real reason to, nobody goes and fights the guy in full plate thinking the fight is going to be quick, that's just not a realistic view. But you also want a couple tougher monsters that can either join the swarm with their multi-attack, or spread themselves among the rest of the party to add urgency to the swarm getting dropped and freeing the tank up, and then one big bad who never even looks at the tank until he's free (have them disengage if they have to, the tank being on lockdown away from the big bad is the goal of the structure) but instead focuses on the rest of the party, creating a sense of urgency (the paladin might not be in a lot of danger per se, but his choices are limited and his movement constrained, so his allies are in danger and its the paladin's job to protect his friends). Also, have the big bad ramp up the threat as the paladin struggles to get away from the swarm to keep the pressure on. The paladin is only going to have so many castings of misty step, and those slots are pretty darn useful for smiting, shoving takes an action so he can't shove and then disengage, so he's got to thin the numbers, or take a bunch of at-ops.
Heck, a dragon with a swarm of kobolds (talking like 15 or so kobolds here) is effective for this tactic as well, since the only point is to keep the paladin from the big bad while the party suffers the consequences. Humanoids can pull this off pretty easily as well (thugs, veterans, guards, gladiators, mages, archmages, etc.). You've got monster options for any difficulty of encounter you'd want to make if you've got a theme in mind. I just was using the above group as an example of some creatures that could actually sit in melee with the paladin and keep him locked down for at least a few rounds. If he gets out without killing them, they just move to resurround him as best they can and keep up the multi-attack salvo of paladoom.
There's also anit-magic fields, which I would say suppress smites, auras, magic equipment, etc. So there's that if you don't want to throw super-deadly encounters at your party, but I feel if you're going to threaten a party, you should go whole-hog intimidation fights and then play the monsters slightly suboptimally (totally a suboptimal tactic, the monsters would be best used splitting up among the party with the bearded devils going after casters, barbed going after other melee, and the rakshasa invisibility tanking the paladin). You want the players to think, "Is... is he trying to kill us? Is he bored with this game? But I like my character!" No, no you're not bored with the game or trying to kill them, you're just giving them a gentle reminder that they're not invulnerable.