Here is a attack I brewed up for use in my home campaign to great effect:

Blast of Flame. Ranged Spell Attack: +6 to hit, range 30ft., one target. Hit: 10 (2d6 + 3) fire damage. If the target is a creature of a flammable object, it ignites. Until a creature takes an action to douse the fire, the target takes 5 (1d10) fire damage at the start of each of its turns.

I really enjoy the flavour! Wouldn't it have to be an Arcana check, however, being a planar inhabitant?

Poor thing gets instakilled by Tidal Wave due to its Water Susceptibility cause even a 5x5x5 corner of tidal wave hitting it contains 935 gallons of water thus that's an instant 935 cold damage.  If it takes the full brunt that's a 10x10x10 cube of water due to being a large creature which makes it 7480 gallons of water.

If I am reading Tidal Wave correctly, it is a sheet of water that impacts the described cube (30'x10'x10') but it doesn't specify the thickness of the sheet of water. The spell describes an impacted area but not the volume of water that hits that area. It is not saying that the described cube is completely filled with water, but rather that every creature in that cube is hit with enough water to damage it. 

Watery Sphere, on the other hand, would give you 3917 gallons if you dropped it correctly, so insta-kill is still an option. I don't think my druid will be using this form unless I am certain there are no casters around. 

Gohankuten and Fordan, while I agree with the latter's answer in the first part, the second part is implying that every drop of water hits the Elemental. Like most creatures, the Fire Elemental doesn't actually fill its space's three-dimensional volume, and no matter how you position a Sphere of Water most of those gallons would be wasted. I understand the science of what you're saying and it's super cool at that, but the vast majority of that water is going to be harmlessly blasted away by the steam explosion as soon as the bottom of the sphere hits the elemental... Mind you, I think it would have to depend on the saving throw (which honestly would have to be a thing so that a CR 5 monster isn't easily instakilled by a 4th level control spell). It certainly could be super damaging though!.. Just not three thousand hp damaging.

This was posted on the page for the Tidal Wave spell itself

Quote from rmdeming >>

For those of you who have run into the "How much damage does Tidal Wave do to a Fire Elemental" problem, here are some numbers for you.

1 cubic foot of water contains ~7.5 gallons of water (https://www.ag.ndsu.edu/archive/dickinso/research/2004/range04c.htm). Therefore a 1' wide x 1' tall x 10' deep section of Tidal Wave (10 cubic feet of water) would do about 75 points of cold damage.

An average adult male has 20.45 square feet of body surface area (https://www.calculator.net/body-surface-area-calculator.html). This is the whole surface area, so a little less than half this number would be the front surface area, we'll say ~10 square feet.

If the Fire Elemental were the size of an average human male and hit by a Tidal Wave spell it would take 750 points of cold damage.

So yeah, that Fire Elemental is toast.

that math only works if you're assuming that the entire volume of water affects the elemental as long as any part of that volume of water comes into contact with it.  By that logic, the elemental could touch a pool of water containing thousands of gallons of water and would die instantly.  Also, the reply you quoted says that 10 cubic feet of water would do 75 points of cold damage and that the front surface area of a fire elemental is ~10 square feet, so tidal wave would cause it to take 750 points of cold damage.  That doesn't work at all because, even if you assume that the full volume of water affects the elemental as soon as it comes in contact, having a surface area of 10 square feet means that even by a generous estimate, it can only be in contact with 10 1-foot cubes of water at the same time, so it would only take 75 damage.

• By that logic, the elemental could touch a pool of water containing thousands of gallons of water and would die instantly.
  • False... The tidal wave is moving. Unless the pool of water is moving or splashes, the elemental is only affected by the amount of water it touches.
• it can only be in contact with 10 1-foot cubes of water at the same time
  • True, but....
• it would only take 75 damage.
  • False - The tidal wave is in motion and is 10 ft thick, so the elemental would be in contact with 10 cubic feet of water ten times before the tidal wave fully passes by

but if its only the amount of water it touches then how do we define "touching" one gallon of water?  how do you convert that 3-dimensional volume to 2-dimensional surface contact?  

Just double the damage and be done

Honestly, I believe using that attack should raise its cr and intelligence, there are different levels of elementals. Look forward to seeing it!

Quote from RickAJr >>

Describe the following to your players to provide better immersion - (show, don't tell during their first encounter):  A large mass of ambulatory flame races across the ground, seeming to flicker and spark from a central, humanoid-shaped conflagration. Like a living inferno, the fire-creature’s burning dance of heat and flame brings it ever closer.  Wherever it moves, it sets its surroundings ablaze, turning the world to ash, smoke, and cinders. In addition, you might want to allow them an Intelligence - Religion check to know the following (accumulated - a roll of 25 or above would know all): DC5 (very easy): attacks with two of its blazing flames DC10 (easy): can move through narrow spaces up to 1" DC15 (medium): resistance to bludgeoning, piercing, and slashing from nonmagical attacks DC20 (hard): attacking the creature with fire or poison has no effect - will take minor damage from water DC25 (very hard): immune to exhaustion, grappled, paralyzed, petrified, poisoned, prone, restrained, unconscious - additionally that it has darkvision out to 60'

 I would say the immunity to fire would be a very common and sensible assumption. I don't see why you''ve placed that in DC20.

i love dnd

Considering that a fire elemental is used to power airships; an adventuring party could make some coin capturing fire elementals and selling them to airship builders.

Now I just need some buckets of water to pour on this thing and some flint n steel and ba bam! Nether portal is ready to go.

i am made a fire elemental encounter

you should have seen it lol

This is the best combat option for Circle of the Moon Druids. Just sayin

Ok I’m two years late but I just realised that since wild shape states that all concentration spells still carry on after casting, you can combo this form with Investiture of Ice to completely negate the water weakness. It’s quite expensive, but have fun being completely immune to Fire and Ice

I've just realized that my Circle of the Moon Druid has both these things.  We began a "Christmas Special" of our campaign today, and I discovered that I like burning things.  Like the human-sized nutcrackers that came for us.  THEY BURNED SO WELL.  I'm going to note the Investiture of Ice while we continue our Krampus hunt.  I have a feeling it will come in handy, especially since we have no mages in our party to burn thin- I mean to do the enemy damage, so thank you good sir.