I don't like using both declarations and expressions for functions.

The callback you pass into reduce can do everything, including parseInt

I think the whole case+arr.push statement can be more easily (and more succintly) done with an object that maps certain Cyrillic characters to Latin characters.

Why did you add the backslash character?

I see you use very short variable names (c, e, al).
In most cases it's better to use variable names that clearly communicate what kind of value
they refer to.

I did, but my experience with the katas on this website is that they are self-contained.

Yes.

The only hint is the part where it says it'll have tests for big inputs. That's not very explicit.

Description says: "You always start with the boss with difficulty level === 1".
So there'd never be a testcase where the difficulty start with 2 or 3.

I went for code readability and functional style programming.

Single letter variables, :-(

You need two conditions, in the correct order.
First condition checks whether the key has length 5, if so, add the key to the array.
Second condition checks whether the object has length 5, if so, add the key to the array.