What is the best 2nd level spell to concentrate on to maximize damage?

Question

In a party full of crowd control, the Magus wishes to dish out as much damage as possible. What concentration spell will allow them to do that, under the given circumstance:

• The Magus is 4th level, any race, any class/multiclass, feats available
• The damage is dealt out over 1 minute maximum
• The rest of the party is focused on their own goals, so any preparation or buffs on the Magus will have to come from themselves: (no time limit)
• Assume things go perfectly 100% of the time: ie enemies will always fail on saving throws, the Magus will always succeed on an attack roll, disregard advantage and disadvantage
• The only source of damage tracked comes from the Magus' concentration spell; no need to worry about ensuring their action is free every round to cast another damaging spell
• Assume all damage is average: 1d6 = 3.5 damage, don't round
• If a spell can hit multiple targets, there are 2 available each round
• These 2 enemies behave optimally for your damage: if an enemy must end its turn in the spells area to take its damage, then 2 enemies in a round will do so.

Answer 1

1st place: Spike Growth

Spike Growth creates a 20-foot radius (40-foot diameter) circle which is difficult terrain and causes 2d4 damage per 5 feet of distance that a creature travels through it. Making the assumption that:

• 2 enemies in any given round have to try and move through the area for some reason;
• those 2 enemies spend their entire movement and dash as an action to get through the difficult terrain;
• those 2 enemies have a normal speed of 30 feet, so after accounting for difficult terrain they travel 30 feet each through the area for a total of 60 feet's worth of damage;

Then the spell will deal 24d4 (60) points of damage per round - averaging 600 points over the full minute allowed in your conditions.

You're very unlikely to be able to actually get that performance out of the spell, since it does depend heavily on your enemies making very poor tactical choices, or that you are otherwise able to ensure that they need to keep moving through the affected area - if they stand still or can walk around it, they take no damage.

But, if circumstances are perfect and your foes keep dragging themselves through the thorns, no other second level spell comes close to spike growth's damage. Even if you make the assumption that you only get half the damage out of it and do a total 12d4 (30) damage per round, that's still nearly as good as what you can get out of other optimising other 2nd level spells.

Spike growth is available (at character level 4) to druids, Nature domain clerics and warlocks with the Genie (Dao) patron.

2nd place: Moonbeam

Spike growth's position as number 1 requires a lot of very favourable assumptions about the battlefield and enemy behaviour. An alternative more likely to be viable in practice is moonbeam, which creates a 5-foot radius (10-foot diameter) cylinder that deals 2d10 damage to any creature which starts its turn there or enters it for the first time during a turn.

As per the questions' premise, we'll assume that 2 enemies in any given round will either start their turn in the area or pass through it as a part of their movement during their turns, each taking 2d10 (11) damage for a total of 22 damage dealt per round.

However, that only covers the damage that moonbeam deals on the enemy's turns. If an enemy ends their turn standing outside the area but is moved into or through the area during someone else's turn - i.e. yours - they will take an additional 2d10 damage.

As per the question's premise, we will assume that at least one of the enemies always helpfully positions itself just next to the moonbeam in such a way that, on our own turn, we can push or pull them to force them back into the area. Any caster could achieve that using a shove attack, but a druid could do it more efficiently using the thorn whip or gust cantrips to respectively pull or push the enemy into the area.

Under that assumption, we increase the total damage per round to 6d10 (33) and so deal an average total of 330 damage over the minute that moonbeam lasts.

Above and beyond this, if we're a druid, we also have access to the thunderwave spell, which can be used to push around multiple enemies at a time. If we devote our remaining 6 spell slots (4 1st, 2 2nd, assuming all four levels are in full-casting classes) to casting thunderwave, then on six of those turns we can conceivably force both enemies back into the moonbeam, and so in six of those ten rounds we will actually deal 8d10 (44) damage, and moonbeam's total average damage over the full minute will be 396 points.

(It is important to note that although we can also use an action on our turn to move the moonbeam around, the official interpretation of the rules according to Sage Advice holds that moving the area of moonbeam onto an enemy does not count as the enemy "entering the area", so this cannot be exploited to deal extra damage - to get the extra points out of it, we must rely on forcing the enemies to move, not moving the moonbeam.)

Moonbeam is available (at character level 4) to druids and Twilight domain clerics, but druids have the easiest time maximising its effectiveness by combination with other spells.

Answer 2

(Carcer's spike growth answer is certainly accurate given the criteria allowing the assumption of two targets running back and forth. I believe this would be the next best if we assume nothing more generous than the targets standing still.)

Moonbeam, available to any druid or to Twilight domain clerics by the question's restriction of character level 4.

2d10 damage per target, so:

• 11 average damage per target per round
• 22 per round with the two targets provided by the question
• 220 damage total split between the two targets over the 1 minute timeframe given in the question

This is the same amount of damage as dragon's breath if the dragon's breath caster were to additionally spend an ASI on an appropriate Elemental Adept feat to pair with it- that feat changes the average damage of dragon's breath d6 die to 3⅔, for the same 11 per target/22 per round. (There is no appropriate version of that feat for moonbeam)

Moonbeam also has the perk of requiring no action to do continued damage on following turns if the initial targets remain stationary.

Answer 3

I can't beat the theoretical damage of Carcer's answer, so I'll make the assumption of not masochistic enemies that drag themselves through the thorns.

Dragon's Breath

If you play a sorcerer or wizard you can cast Dragon's Breath on yourself which can breath an element of your choice at enemies for 3d6 damage per target. Assuming two targets fail their save as given by the question, that is 6d6 = 21 per turn. That is 210 damage over 10 turns or 1 minute.

You do have to get a bit close to enemies but not into meele so you can still back up with any remaining movement.

A bit extended out of scope for the question:

• As a sorcerer you might be able to twin the spell to have two people doing that damage. That would be 420 damage for your spell slot. (Twinning this spell is a controversial topic here. Some think it's fine, Jeremy Crawford thinks it's not, so ask your DM). As @Carcer noted in the comments if you have a familiar (e.g. via ritual caster feat) you can do that double damage all by yourself. Again, better clarify with your DM first.

• As a divine soul sorcerer you could cast spiritual weapon as a bonus action on your second turn for an additional 1d8+4 (assuming you got your charisma to 18). That is 8.5 damage or 76.5 damage over the 9 turns for which you have your dragon's breath active. This does not require your concentration.

Answer 4

Dragon's breath

The spell Dragon's Breath does 3d6 damage of a type of your choosing out of five options. Provided you have enough enemies that can be hit by a 15 foot cone attack you can score a lot of damage with it per turn. If your targets are vulnerable to one of the damage types you can select you'll get even more damage from it.

This, of course does require your enemies to be within 15 feet and your damage total will depend on how many you can hit at once.

Damage per turn will be 10.5 per target assuming saves are failed or 21 damage per target if they are vulnerable to the damage type.