Your attack lands and seems to somewhat damage the mephit, but it is still standing. 

Bot-ing for Ezrer: 

Ezrer will cast firebolt on the same mephit. 

To hit: 21

Damage: 1 

It is then the mephits’ turn. They both exhale 15- foot cones of cold air, encompassing every member of your party. Everyone must make 2 seperate dex saves.

Dec saves: 21 and 8

Ulysse dex saves: 7 and 8

Dex Saves: 18 and 12

Dex Saves: 10 and 20 

Frost breath damage: 

Boil: 7

Ulysse: 9

Jahn: 4

Doter: 4

Initiative Order:

1. Ulysse (21) 

2. Ezrer: (19)

3. Ice Mephits (18)

     |   Mephit A (has taken 2 damage from produce flame, and 1 damage from firebolt

     |   Mephit B (undamaged)

4. Jahn (14) 👈 <you're up>

5. Boil (10)

6. Doter (9)

Jahn would rush over and attack Mephit A with their battle axe. 

17 to hit for 9 slashing damage. 

Quote from Chicken_Licken >>

Initiative Order:

1. Ulysse (21) 

2. Ezrer: (19)

3. Ice Mephits (18)

     |   Mephit A (has taken 2 damage from produce flame, and 1 damage from firebolt

     |   Mephit B (undamaged)

4. Jahn (14) 👈 <you're up>

5. Boil (10)

6. Doter (9)

 Are the ice mephits vulnerable to fire damage?

Quote from Engineered_Beard >>

Quote from Chicken_Licken >>

Initiative Order:

1. Ulysse (21) 

2. Ezrer: (19)

3. Ice Mephits (18)

     |   Mephit A (has taken 2 damage from produce flame, and 1 damage from firebolt

     |   Mephit B (undamaged)

4. Jahn (14) 👈 <you're up>

5. Boil (10)

6. Doter (9)

 Are the ice mephits vulnerable to fire damage?

 You don’t know. 

(Still waiting on Jahn to do their turn)

Quote from Stivali >>

Jahn would rush over and attack Mephit A with their battle axe. 

17 to hit for 9 slashing damage. 

 (Happened right before someone asked about the fire vulnerability.) 

Quote from Silvercoins >>

Quote from Stivali >>

Jahn would rush over and attack Mephit A with their battle axe. 

17 to hit for 9 slashing damage. 

 (Happened right before someone asked about the fire vulnerability.) 

 (My bad, I didn’t see that, thanks)

Quote from Stivali >>

Jahn would rush over and attack Mephit A with their battle axe. 

 

17 to hit for 9 slashing damage. 

 Your attack hits, and but the mephit is still standing, albeit barely. 

Boil, your turn. 

Occ: can I use destroy water on these creatures? 
Destroy Water. You destroy up to 10 gallons of water in an open container within range. Alternatively, you destroy fog in a 30-foot cube within range.

If not, then Boil is going to cast Guiding Bolt with a 16 to hit dealing 13 damage to the second Mephit and going to use Fury of the Small as a bonus action on the Mephit that has damage already dealing 1 damage.

Quote from BoomBeard >>

Occ: can I use destroy water on these creatures? 
Destroy Water. You destroy up to 10 gallons of water in an open container within range. Alternatively, you destroy fog in a 30-foot cube within range.

If not, then Boil is going to cast Guiding Bolt with a 16 to hit dealing 13 damage to the second Mephit and going to use Fury of the Small as a bonus action on the Mephit that has damage already dealing 1 damage.

 

 They are not technically in a container, so no, you cannot use destroy water on the ice mephits. Also, they are the same size as you, so you cannot use fury of the small. 

So, your guiding bolt strikes true on mephit B, and although it doesn’t destroy it, the next attack roll will have advantage. 

Doter, your turn. 

Doter will move to Mephit B and smack it twice with his claws (unarmed strike). 

Attack: (Rolled twice for Advantage 12, 15) 15 to hit and 4 for damage 

Bonus Attack: (Rolled nat 20) 25 to hit and 10 for damage 

Quote from Silvercoins >>

Doter will move to Mephit B and smack it twice with his claws (unarmed strike). 

Attack: (Rolled twice for Advantage 12, 15) 15 to hit and 4 for damage 

Bonus Attack: (Rolled nat 20) 25 to hit and 10 for damage 

 (It is destroyed) How would you like to do this?