CSS Selector "(A or B) and C"?

For those reading this >= 2021:

I found success using the :is() selector:

*:is(.a, .b).c{...}

Not yet, but there is the experimental :is() (formerly :matches()) pseudo-class selector that does just that:

:is.a .b) .c {
  /* stuff goes here */
}

You can find more info on it here and here. Currently, most browsers support its initial version :any(), which works the same way, but will be replaced by :is(). We just have to wait a little more before using this everywhere (I surely will).

is there a better syntax?

No. CSS' or operator (,) does not permit groupings. It's essentially the lowest-precedence logical operator in selectors, so you must use .a.c,.b.c.