Okay, I was challenged in a different thread by DxJxC to make a party combination of level 4 party that can do over 300 damage in one surprise round. Here are the requirements:

1. There can be no more than 6 party members.
2. They have to be level 4.
3. There are only allowed the following magic items; 1 rare magic item, 4 uncommon magic items, and 6 common magic items.
4. No Unearthed Arcana or Homebrew
5. Assume that each attack hits, and do average damage.
6. Critical hits aren't automatic, they have to be forced, by an ability that allows you to crit in certain circumstances.
7. The monster will be a humanoid, that goes after everything else does, and has more than 300 health. It has no resistances/immunities/vulnerabilities.
8. No actions against the monster can be taken before the round, no Hold Person, Hex, so on. Everything hostile must happen on the same turn as the damaging round.
9. You may use any abilities/magic items that benefit you before you go.
10. You may hide.

My main idea is to have the party have the following:

6 Level 4 Divination Wizards, with Hold Person and Jim's Magic Missile. When Jim's Magic Missile is cast at 2nd level, it does an average of 50 force damage with Hold Person. This will get us to the bare minimum damage.

Can you beat this? I did it with no magic items.

okay, so here is my take

you have one person for the setup: a changeling valor bard with 20 charisma (using standard array/point buy for 15+3(changeling)+2(asi level4)

and then the rest ist absolute crit nova madness: half-orc gloomstarker rangers with the Great Weapon Master Feat. (16 STR)

i am not sure if some preperation is allowed: if they have some time/a few rounds to prepare, all of the rangers will have cast hunters mark on the target, and each one will have a bardic inspiration from the setup guy (ill do my calculations with and without preparation)

lets look at one Gloomstalker and his attacks with a Greataxe:
He has one attack with his action, an additional one with his action because of Dread Ambusher (Gloomstalker) and since all hits will be crits he will get a third attack with his bonus action.
each of these 3 attacks deals:
1d12 (Greataxe) + 1d12(Greataxe Crit) + 1d12 (Savage Critical from Half Orc) + 3 (STR mod) +10 (Great Weapon Master) = 3*6.5+13 = 32.5 dmg
[if they had time to prepade, we add an extra 1d6(Hunters Mark) + 1d6(Hunters Mark Crit) + 1d6(Combat Inspiration) + 1d6(Combat Inspiration Crit) = 4*3.5 = 14 dmg for 46.5 total]

the additional actin attack from gloomstalker gets an extra 1d8 dmg, doubled by crit for an extra 2*4.5 = 9 dmg

total average damage per Gloomstalker is now: 3 * 32.5 + 9 = 106.5
[or, if we had time to prepare 3 * 46.5 + 9 = 148.5]

so with potentially 5 Gloomstalkers and one Bard thats there for emotional support and guaranteed criticals, assuming all attacks (which would be at advantage with +0 to hit) hit thats an average of
5 * 106.5 = 532.5 dmg

[or 5 * 148.5 = 742.5 dmg, if they had time to all be inspired and to cast hunters mark on the enemy in advance]

in conclusion:

... half-orcs with guaranteed crits are crazy

Edit:
I just realized Battlemaster Fighters to pretty much the same thing as Gloomstalker Rangers, using action surge and any maneuver that adds the d8 to damage, but with them you get a d8 to all 3 attacks instead of only 1. and you get the improved fighting style (no hunters mark preparation though(

this means every hit deals
3d12 + 2d8 + 13 [+2d5 inspiration if prepared] = 41.5 dmg [48.5]
with 3 attacks per each of the 5 fighters for a total of 622.5 dmg [727.5 prepared(lower because no hunters mark]

Fighters also get the Great Weapon Fighting - Fighting Style which rerolls 1s and 2s on their Greataxe, increasing the average dmg from 6.5 to 7.33
I am unsure if this is also meant to apply to the extra d12 from the critical and/or the half-orc savage critical (seen both interpretations).
accounting for the fighting style would increase the dmg by 0.833 for each d12 it applies to, so the total damage increases by
0.833 * 15 = 12.5 (or 25 if it applies to two d12s (or 37.5 if it applies to all 3))

not that much of a difference, but thats still up to 634.5 dmg now, using the worst assumptions.

I'll put this here. This will clear up some of the rules, I hope.

Quote from DxJxC >>

I did originally mean 1 round. At the very least, spells and effects that noticeably effect the target are not set-up. There is no such thing as a precombat hold person, that is a hostile action.

Effects on yourself are fine (I guess). And you can hide pre-combat. No weaknesses or resistances. Lets make the scenario: you know when and where the enemy will be and combat starts the moment you have line of sight.

You also changed a restriction in your post. I said to use weighted damage for crits and misses assuming 10 AC (and just to throw more numbers in, lets assume +0 on saves).

Hold person and Jim's magic missile is a good combo though, I'll have to remember that one.

 so here is a very ridiculous approach:

still one guy using hold person, but this time the dps guys are.....

multiclassed Paladin 2 / Moon Druid 2. (multiclass spellcaster level 3, so they have two 2nd and four 1st level slots)

These Guys can wildshape into a deinonychus which can potentially do 4 attacks per round when charging an enemy.

(only 3 if you assume they could not wildshape in preparation, so they are missing their bonus action)

And since Class features can be used while in Wild shape you can use divine smite on each of those attacks.

so without preparation we have 3 attacks, first 2 with 2nd level spellslots used for divine smite, the last with only a first level slot. for a total of

3d8 (weapon attacks) + 6d8 (two 2nd level smites) + 2d8(one f1st level smite) + 6 (str-mod times 3) = 11d8 + 6
with dice doubled for crits thats 22d8 + 6 averaging to 105 dmg 

with one bonus action pre-combat to transform you get an additional bonus action attack for an extra 3d8+2 for a total of 14d8 + 8, critical for 28d8 + 8 = 134 dmg

with a second bonus action to prepare they could all cast divine favor and have an extra d4 on every weapon attack for 28d8 + 8d4 + 8 = 154 dmg

... and thats per divine smiting dinosaur

so with a few bonus actions to prepare, a spellcaster that does the hold person and 5 angry holy velociraptors thats a whopping 770 dmg (o.o)


... also its 4 am and i should sleep

 My original setup was going to be paladin 2/war cleric 2 (gwf) with thunderous smite set before hand. With +1 greatswords and 2 level 2 divine smites. That would be 56.16 (6d8+4d6+8) ×6 = 337 damage.

I originally proposed using weighted averages based on hit and crit % for 10 AC. Which drops my idea to 52.55 per PC and 315.35 total. And I never even considered hold person and half orc (which does crazy things to the numbers).

Hold person, half orc, and great weapon master is a great combo. Gloom stalker dread ambush or fighter action surge for an extra attack, and you could clear the challenge with a standard 4 person party.

And the druidin deinonychus is even more amazing. More amazing than even you calculated when you consider that wildshape keeps racial traits too so keep half orc savage attacks, so that is actually 172 damage per favored smitey dinosaur. Add insignia of claws and combat inspiration (because why not) bring hit to +5 and damage to 183, for a weighted average damage of 153.72 per dino. A party of 4 should be able to do 400+ damage ay level 4.