Ideally, Sacred Flame kills it (roughly) twice as fast
A simple formula answers this, with X being the number of rounds needed to kill the Shadow.
1d10 averages 5.5, halved is 2.75
1d8 averages 4.5, doubled is 9
For Firebolt, you hit on 8-20 (chance for crit), so .65 x (5.5/2)(crit bonus)x(XFB) >= 16
(Need to hit AC 12; roll 1d20 +2 Int +2 proficiency)
(14 / 13 = crit factor; (x+1)/x for damage with the chance of crit thrown in)
For Sacred Flame, you hit on 13-20 (and no crit) .4 x (4.5 x 2)(XSF) >= 16
With a DC of 11 (you only have +1 due to your 12 Wisdom)
(The Shadow makes a dex saving throw versus the spell)
Solve for XFB: 16/(.65 x 2.75 x 1.077) = 8.31 rounds
Solve for XSF: 16/(.4 x 9) = 4.44 rounds
As a practical matter, being able to kill it in ~4 rounds rather than ~8 rounds means that your Sorcerer with that meager 16 HP has a far better chance to survive the fight before having no offensive capability.
Actions
Strength Drain. Melee Weapon Attack: +4 to hit, reach 5 ft., one creature.
Hit: 9 (2d6 + 2) necrotic damage.
Two hits and your Sorcerer is down.
This is an encounter were the Cleric's "turn undead" would be a handy skill to have.
This whole problem also presumes that you stay more than five feet away from the Shadow. In the case of firebolt, since that is a ranged spell attack rather than a spell that calls for a save, if the Shadow closes to melee range your firebolt attacks at disadvantage.
(SRD p. 103; spell attacks) Most spells that require attack rolls involve ranged attacks. Remember that you have disadvantage on a ranged attack roll if you are within 5 feet of a hostile creature that can see you and that isn’t incapacitated.
The best I could come up with is 2,084 damage.
Use an Oathbow (1d8+3d6)
Drink a Potion of Giant Size from SKT (Oathbow now deals 3d8+9d6)
Use an Arrow of Slaying (+6d10)
Have Dexterity 20 then use Dex Tome (+6 dmg)
Paladin 3 - use Branding Smite (+2d6) also gives you Martial Prof to
use the Oathbow
Rogue Assassin 17 - get "Assassinate" "Death Strike" and 9d6 Sneak Attack. Get
Sharpshooter feat while you're at it. (auto-crit and double damage,
+9d6 +10)
Have a Battlemaster Fighter use Commander's strike on you (+1d12)
Have a Red Dragon Sorc with 22 Charisma cast "Flame Arrows" on you (+4d6 +6)
Have a Valor Bard cast "Elemental Weapon" on you and give you Combat Inspiration (+3d4 +1d12)
Have a Paladin cast "Magic Weapon" on you (+3)
Have a Grave Cleric cast "Holy Weapon" on you and use "Path to the Grave" on
the target (+2d8, vulnerability to attack)
Have a Wizard cast "Enlarge" on you (+1d4)
So that works out to:
5d8 + 18d6 + 6d10 + 4d4 + 2d12 = 40 + 108 + 60 + 16 + 24 = 248
Doubled for critical hit = 496
Damage bonuses total = +25 = 521
Double that for Death Strike = 1,042
Double that for Path to the Grave = 2,084
If you rule that the Potion of Giant Size doesn't increase the Oathbow's extra damage, then the 18d6 becomes 12d6 and the total is adjusted to 1,796.
I don't think there are any other questionable rulings in the calculation.
Best Answer
The maximum average damage for a single unarmed strike is 622 damage
(or 478 with unfavorable rulings on Divine Smite and banishing smite, see addendum)
The Character
The character, let's call him Connor, to pull off the most damaging unarmed strike is a level 20 lizardfolk with the following classes:
...and the following equipment:
...and:
The Helper
Our helper, let's call him Hunter, is a level 20 character with the following classes:
and the following equipment:
The Preparation
Before the unarmed strike, a handful of buff spells must be cast:
The Strike
After activating the Spell Glyph, Connor's unarmed strike has the following modifiers:
Resulting in the following calculation:
This totals to an average damage of:
$$(2*3.5+2*6.5+11 \text{ slashing} +10*4.5 \text{ radiant} + 18*3.5 \text{ psychic}$$ $$+10*5.5 \text{ force}+ 2*4.5 \text{ necrotic} +18*6 \text{ lightning})*2=622\text{ damage}$$
Relevant rulings:
Since Bite includes a natural weapon for use with the unarmed strikes:
...Divine Smite works with the lizardfolk's fanged maw, even though it doesn't work with normal unarmed strikes (see the second section of this answer to a related question for details).
If your GM still rules that Divine Smite does not work, simply be an aasimar instead and replace the paladin levels with bard levels (with the Boon of High Magic epic boon). Then cast wish replicating the effect of glyph of warding (Spell Glyph: alter self to get a +1 natural weapon that can be used for unarmed strikes) for 574 damage instead. That is still more than enough damage to one-shot the Kraken, so the extra radiant damage is just insult to injury.
Psychic Blades should work with any unarmed strike, as unarmed strikes are considered weapon attacks
Banishing Smite should work with any unarmed strike RAW, as even though your weapon is what is crackling with force, the weapon attack is what gets the extra damage, and we've established with the Psychic Blade ruling that unarmed strikes count.
There is similar RAW instance regarding whether the weapon is the source of the visual effect, but not the damage with the current wording of holy weapon, as explored in this question. However, for our purposes, we are using a natural weapon that is specifically allowed to make unarmed strikes, so banishing smite works anyway.
If your GM overrules this and deems that a weapon that is not a natural weapon is required, replace banishing smite with wrathful smite for a loss of 96 damage.