Randomus is not that hard to take down unless he can set up trick room. Just make sure that you can get rid of Reuniclus ASAP. Your team and reserve are looking good to take on him but you might want to bench Noivern and switch to Zoroark for this fight since the field effect give psychic type move gain a rock type so Noivern gonna get wrecked(Trust me on this Noivern is my ace too). You might want to try Persian from my playthrough oddly enough he wreck Randomus team pretty horribly(Techinician+Fake out+Assurance = RIP Reuniclus). About Leavanny, I don't think it's not a great idea due to how his field work. Don't get me wrong Leavanny is pretty good pokemon to train but just not for this fight.