How the wizard killed a cr 13 elemental with a single spell

This is my first year DMing, and this is a hugh fun, i started DM after a oneshot with this group of new players when I was the only veteran, after the oneshot ended they all had so much fun we agreed i will start a campaign online through discord, I started planning, they created characters and we begun playing. It’s been going on for almost a year, and now the players arrived to an island full of elementals as part of the quest, I prepared for this island for months, creating what i called; dual elementals, but all of them have unique abilities, they are not just combinations. The specific one in this story is the air-fire combination which i created to be a hugh fire tornado elemental, the players encountered it just as a new players was about to join, this was the entering session, and her rouge’s group was slaughtered by that elemental, they landed, with their flying carpet and ran on to help, the wizard found out the rouge was someone he knew from his past, we tied their backgrounds as the rouge asked before, and soon after the combat begun, 5 players at level 11 vs 3 air elementals and one hugh fire tornado at cr 13. All the players rolled high initiative, and acted before the elementals, the monk and the rouge flanked one of the air elementals and oneshoted him, then came the wizard, which in the weeks before they entered the island dedicated much of his time to study weaknesses of the elementals so he knew water are good against fire, obviously, so he casted tidal wave. Tidal wave works in a way that makes a hugh amount of water to spill on the targets, usualy it does normal damage but since this is part fire elemental, and it is hugh it has alot of areas to get hit by gallons and gallons of water, and the fire elemental has an ability that says, for every gallon of water spilled on it, it takes 1 cold damage. So when he casted it, my face froze, i was NOT rdy for it, from my prespective he crushes the wave on the elemental so the full cube of each space the elemental occupies is filled with water, after the math is done, the elemental space was hit by 16831 gallons of water, which means he takes extra 16,831 points of extra cold damage, simply amazing, the other elementls ran after that and the battle was won, with a single spell destroying a cr 13 monster. 

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