The Elemental in the Shield Maze prompts a tutorial message that "you don't have to try every fight, some side encounters will push your shit in" message, so it's not like the game fails to warn you about such encounters.
Couple things:
-This fight is an example of how Owlcat fundamentally fails to understand the system they're running. A CR 8 fight isn't supposed to be winnable by *any* level 2 party of *any* composition. I was able to win through pure luck--it rolled poorly to hit and I scored some critical charges. But in any normal game that's a TPK. It's like "rocks fall everyone dies" but the DM actually goes to the trouble of rolling the rock damage, and somehow rolls 20d6 with all 1s.
-What is the actual point? The game has some blatant unfair fights. So you should... just... not... fight... stuff... ? Okay? But why? It's not even like the elemental is guarding anything good or gatekeeping a tougher area of the dungeon. it's just a random out of depth monster because fuck you that's why.
The DM has the power to obliterate the party at any time but no one is impressed when this happens for no reason.