Riolu gets dig and fighting moves which are good coverage, especially in PMD where dig has such high power for some reason. With shinx you get electric coverage.
However you may have problems with psychic and ground types that have super effective moves, there are also ghost types which are annoying because they travel through walls and can still hit you.
I suggest you get either a dark type Pokemon and teach it pursuit, or get a grass type and teach it bullet seed because that move is broken.
Hope this helps.
I know you probably already beat the game by now but I wanted to give this question as answer.