Could you explain a little better how your party would overcome the teleporting and ranged capability?
Let's say we star the encounter at traditional range, somewhere in the 50 foot range to start. I call for initiative. The Lore bard subtracts 1d10 from Maloch's initiative and both the ranger and the barbarian have advantage on initiative.
I assume the Barbarian goes first, but really the Ranger or him could and it won't change anything for reasons I'll explain. Barbarian rages, closes the gap, and lays in with 2 reckless attacks with GWM against a AC of 19 (meaning it's way more likely he hits than not), rerolling 1 poor damage roll. God help the devil if he crits. The devil teleports 150 feet away...or tries to but it's counterspelled by the illusionist wizard.
The ranger makes 2 attacks with Sharpshooter against her favored enemy with an AC of 19. With a + 1 weapon, at worse, she hits once dealing 1d6 + 16 plus her favored enemy bonus (which I don't have handy), more if she uses her thundering bowstring ability to add an additional 2d6 hit points (she can do this 3 times a day). The devil finally teleports away.
It's the BM fighter/paladin, lore bard, or Illusionists turn next. For sake of this argument, we'll say it's the BM Fighter because he's the most useless. He declares his attack action and his bonus action to grant the SS ranger her 3rd attack of the round and moves to protect the lore bard who is really the lynch pin.
The lore bard then goes and gives out one of her inspiration dice to the ranger because duh. She then casts bless because duh. Her reaction is now up for cutting words again should she need it. The demon takes his third legendary to drop a stinking cloud on either the bard group or the ranger because the players are smart and don't cluster as they are good tacticians. I assume we try to take out the bard and drop it on her group. She likely fails her save, dropping bless, and the fighter likely makes his.
The Illusionist goes next. I have no idea what his plan is, but it probably involves casting haste on the ranger, because duh.
Maloch finally gets to go and regenerates 20 hit points. He is still most likely around 150 hit points at this point and given he's a demon lord decides screw this fight, I can't win, I'm fleeing. If he's fighting on the material plane and decides to fight to the "death", he already has his concentration spell up, so he's rather limited. We'll assume he teleports next to the barbarian or ranger and breathes his breath on them. Most likely, he does this to the barbarian unless the ranger has spent her bardic inspiration dice somehow. The barbarian likely fails the save unless one of his allies give him inspiration (which they will, because duh). Still, for arguments sake, we'll assume he fails.
Round 2: Barbarian goes and has to flee and gets hit with an OA. He may or may not make his save. The monster attempts to teleport away, but nah. Illusionist stops it.
Ranger goes and lights him up with 3 more attacks. At this point, I assume Maloch has sub 100 hit points. He teleports next to the Wizard as a legendary because the Wizard is now hampering him.
BM Fighter goes and Action surges to get next to him. He gives 1 attack to the Ranger (her 4th attack this round) and spends his second attack to attack Maloch. I assume he still has some spell slots left and will land his hit. He then channels his smite into Maloch. OUCH plus no more regen. Things are looking grim. At the end of his turn the monster teleports away never really getting a chance to land a solid blow on the wizard.
Lore bard goes and fails her save again and is looking bad. She moves away from the cloud and casts cure wounds on herself, probably at around 5th level. She grants another bonus die to the ranger.
Illusionist goes and casts lightning bolt on Maloch at say 5th level. We assume the devil saves (or uses legendary resistance) but still takes 10d6/2 damage.
Maloch gets to go again and is in a world of hurt. No regen this round and hovering around 50 but thankfully his breath recharges. He teleported somewhere still within 30 feet of the illusionist/fighter and lights them up with his breath weapon. We assume they both fail and both flee. Haste is dropped, putting a crimp on the ranger's day.
Round 3:
The barbarian gets another save and we assume he fails again. Maloch teleports next to the bard.
The ranger's turn gets skipped. Maloch attacks the bard probably with his bite, but she cutting words it. 50/50 chance he hits.
The BM fighter goes and while still frightened, he takes an attack action to grant the ranger an attack this round. She likely hits given the superiority die. Maloch is probably sub 20 hit points.
The lore bard goes next and is in actual trouble. Bard disengages and hands out another die to the ranger. She's out of dice though, now and Maloch responds by whipping her back in and probably putting her into unconsciousness. Or, he drops another stinking cloud mage.
Illusionist goes and casts another Lightning bolt, this one at 6th level dealing 11d6/2 damage. Maloch is probably dead.
So, I was wrong, 3 round fight if everything goes perfectly for Maloch.
