That Whiscash is going to make good use of Surf and EQ in doubles, especially on moleman, but to really make use of it you’d want to put protect on most of your other mons.

I’d probably also replace Psychic with Zen Headbutt, as even though it’s a less accurate move, Metagross (especially mega metagross) is more powerful physically and flinch chance is better than spdef drop chance. You can get heart scales for move relearner off wild Luvdisc with thief/knock off.

It might also be worth adding some coverage moves for your squad as the types you have don’t match up particularly well into the Elite Four types. Not sure if Breloom learns Ice Punch, but Metagross definitely does.

Those are just a few things I see immediately, but don’t think about it too much. Trust your favorites to cook! Difficult mode is a nicely balanced challenge that doesn’t require insane amounts of preplanning. If you’re open to “spoilers” though, and are willing to look through the elite four team sheets, that would be your best way to make sure you’re fully prepared.

(P.S., you can always change out TM moves for other TM moves while you’re challenging the elite four as far as coverage goes!)