There are many fights that casuals can't do.

In general; all the extreme primals, twintania and all of second coil. Fortunately, extremes are not tied to story so they can enjoy the game by beating story mode versions. Twintania kind of makes it tough but eventually with some decent FC members even casuals can down, or get carried, through it. This argument is foolish otherwise.

Although, it's as OP said, even with more gear if you can't do the mechanic, you can't do the mechanic. Which is what they wanted SE to move away from even a little bit.