I'm not really good at competitive battling myself, but I can give you some small advices on some minor mistakes or not optimal sets you have made:
1. A focus sash lead with belly drum is not working, because either you use your focus sash because of a hit and have 1 hp so belly drum wont work or u use belly drum and the focus sash will be useless. Maybe change for shell smash?
2. Dream eater is (mostly) a bad move in competitive battling. Maybe psyshock would be good.
3. Whimsicott wont be able to use light screen, because the assault vest lets him use only moves that do damage. I would suggest to use a different item like leftovers and prankster as the ability with attacks like substitute, tailwind and leech seed to get a support mon with priority attacks due to prankster.
4. Maybe Specs for Drampa? I think in more games specs would be a better option.
5. Chandelure doesnt have will o wisp, so i think shadow ball would be better. Except you focus on your toxic spikes.
6. Mega Banette. I think destiny bond, shadow claw and knock off are better than shadow force, hex and gunk shot.
Hope i could help you