Its a table-specific thing, really.I personally like epic, over-the-top results (PCs or otherwise). I am not too concerned about balanced encounters. That said, most encounters are balanced and bland, so when something like this comes up, huzzah! In PotA this is really an issue when going against "fire", or about 1/4 of the adventure. Let it ride, in my opinion.
But if you are concerned with an "auto-win" spam of tidal wave, then the next best approach in my opinion is to always say "YES, BUT..."
Create an obscuring fog that makes it possible for the enemy to retreat, or sneak up on PCs or what have you. Even more devastating: if near a place of high concentration magic water or fire, maybe STEAM mephits appear attacking everyone on sight within the obscuring fog...
When in doubt, think "what would J.J. Abrams or Michael Bay do?" (WWJJAD)
Another viewpoint is Tidal Wave vs. Fire Elemental is no different than Banishment Vs. Fire Elemental. It's not game breaking.
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