So what are the details? Here's where I am at:
- Sorcerer
- Innate Sorcery (level 1) to give yourself advantage on all attack rolls for spells for 1 min, 2+/day (or +1 DC)
- Metamagic (level 2) - Empowered for rerolling damage, and Seeking for misses
- Draconic sub (level 3), maybe lightning, +HP, +AC, Chromatic Orb for free
- Fey-Touched feat (level 4) for +1 CHA and Hex
- Draconic (level 6) gives +CHA damage to lightning spells, resist lightning
- +2 CHA (level 8), for 20 CHA
So at level 9, let's compare using your single 5th level spell slot against 5 targets.
Pre-cast Hex (duration is 1 hr, uses concentration) and
Innate Sorcery, then
Chromatic Orb.
- +9 to hit and advantage on each roll, with Seeking spell to help with any misses (1 in 8000 chance to critically miss x5)
- 7d8+CHA lightning + 1d6 necrotic damage per hit = average 40 damage
- max of 5 targets if you can keep getting doubles on 7 dice, which you have a 98% chance of doing (and a reroll for the 2%)
- 9.75% chance of crits (can't use seeking for crit fishing) for +35 average damage per hit, so roughly 43.4 average with crits
- x5 = 217 total average damage before misses
- Assuming a rough average of 17 AC for CR-appropriate enemies, that means hitting about 97.3% of the time! It's insanely hard to calculate average damage since any miss negates the possibility of future rolls, but the maximum average would be 211, and the true average is lower.
- minimum damage is 0!
Or: pre-cast Innate Sorcery (+1 DC), then one
Fireball (10d6 = 35 avg per target, 175 on 5 targets) or
Cone of Cold (8d8 = 36 avg, 180 on 5)
- similar per target damage, but both of which are subject to saves (assuming DC 18 with Innate Sorcery)
- Assuming a rough average of +3 on Dex saves for CR-appropriate enemies, that means ~25% chance for saving for half, or 87.5% total damage, for 30.6 avg x5 = 153 on Fireball and 31.5 x5 = 157.5 on Cone of Cold.
- minimum damage is still 1/2 of the maximum!
Compared together, I feel like Chromatic Orb pulls out ahead, but only because of Seeking spell and Innate Sorcery. Guaranteed advantage PLUS a third chance to roll if you miss gives a huge hit rate. (Seeking says you can use it even if you've used a different metamagic, which might mean it can only be used once; I am unsure.) I ran the numbers without Seeking spell and that's a ~91% chance to hit. Even if Seeking can only be used once, that's still between a 91% and 97% chance to hit 5 times, and almost a 10% chance to crit each time, which save-spells cannot do.
Meanwhile, you're virtually guaranteed to bounce: this is even more likely than hitting, which I didn't expect, and Empowered spell may be wasted as the chance is already so high. The odds that you can get the full damage are really quite good, better than I suspect people realized, and they are much better than the full damage with save spells, even against fewer targets! The only time fireball comes out ahead is if there are at least 2 more targets than you can get with Chromatic Orb! AND Chromatic Orb is like a free Careful spell.
