The spellcaster (Wizard) used magic missile. The cleric just healed and buffed.
I did make it so that the Balor dispelled there Protection from Evil in a 30ft radius. Which of course meant I could actually hit them.
I was doing good damage.
But I think Shidaku is right. I just need to give my creatures more actions per turn against such a large party.
Something I just noticed here. Why would the Balor need to dispel the protection from evil so he could hit them? Protection from Evil only affects one PC and it's concentration. And, it only inflicts disadvantage. Ok, so, I don't target that PC, but, instead, target the cleric. Why did he need to dispel anything?
But, looking at the example of the 19th level fighter, I wonder why these calculations are actually useful. That character has no ranged attacks. Why isn't the balor flying lazy circles 30 feet above him, hitting with the whip, pulling him in the air, hitting him with the sword and then dropping him. The balor in the example should take no damage at all. I guess if the fighter started readying attacks, but, then the balor just uses only the whip, doesn't bother pulling him and still wins.
If the fighter has a +1 bow, so what? He's a strength fighter (girdle of giant strength after all) so he's likely got a 14 or less Dex. He's a heavy armour fighter so he probably dump stated Dex. The fighter's now doing d8+1 damage on attacks. The Balor creams the fighter.
And this brings me back to encounter design 101. Why is the balor fighting in an empty chamber? Is it really that unreasonable to have, oh, i don't know, a 30 foot pit in the room where he tortures his victims? Poof, pull PC's into the pit and they're out of the fight for a round. Hey, how about a couple of big burning braziers? Tip it over, fill the area with smoke, causing the PC's disadvantage on attacks due to concealment, plus possible choking damage. Or, how about, just maybe, a ledge or two somewhere higher up that the balor can use to attack from? Hey, how about a big bubbling hot mud pit or a boiling hot spring? I mean, good grief, it's the lair of an extremely powerful being that's been hanging around in that area for quite a while
These aren't adding XP to the encounter, this is basic dungeon dressing.
But, yeah, in an empty chamber with a 10 foot ceiling? Sure, the demon's got a problem.
---Edit to add
See, I'm really having a tough time with the math here. We've got a 7th level paladin standing toe to toe with the balor - after all it's the paladin's critical hit that kills the balor. 7th level paladin had (10+6*6=46 HP plus, we'll be generous, 14 con =60 HP) The balor attacked the paladin. Presuming one hit with each weapon, that's 21+13+15+10=58 damage, plus the 10 the paladin did to himself with that critical hit, so 68 damage. The paladin died with his own attack and is now ground zero for the blast. No saving throw because he's unconcious, he takes 70 damage. He flat out dies. Never minding anything else, how did the paladin survive? Note, of the other 5 characters, 1 is casting magic missile, so, 15 damage per round, 1 is only healing so, no damage, and the other 3, presumably, are attacking.
How in the hell did they do 262 points of damage in 3 rounds? Actually, less because in the first fight, the balor gets away. Any of the fighter types, assuming Action Surge, dump 40 points of damage into themselves for the 4 hits. Again, how are they not killing themselves? Even assuming 20 points per hit, which is absolutely ridiculous for a 7th level party, they're still only running around with 40-60 HP each. That's presuming no one took aura damage as well. How did they attack, then get out of the aura without eating Opportunity attacks? Remember, one attack, given the damage they've done to themselves, is enough to drop the PC's. The fight only lasts like 3 rounds, it's not like a 7th level cleric is healing that much. He can only cast so many spells.
This just does not add up. To kill the balor, they need to hit it 9 times (again, assuming the 20/hit, which is WAY high for a 7th level party, it should be closer to 15 at the most), that means they've dealt 90 points of damage (well, less, you do have magic missile damage, so, let's say 70 damage) to themselves before the balor has done anything.
Of course, any time the balor rolls a crit, a PC drops flat out. 99 points of damage and no 7th level PC takes that.
I'm frankly baffled how a 7th level party could even really challenge this thing.