PSA: You can finish the Mega Emblem Event missions in a private match with a friend/alt
You can do this relatively quickly if you can set this up. You don’t need to play your matches out. When your opponent concedes, you still get credit for the mission, even while you’re setting your first active Pokémon.
Another tip I can give to trainers is you don’t need to make a bunch of decks for each element. What I did was use the auto build feature for each element. The decks don’t have to be viable AT ALL because your opponent is going to concede after the game loads anyway. After the concession, go back to building your deck, use the auto build, pick another element you need for your mission, then hop back in and repeat.
This should take you about 10-15 minutes to do. No deck building required and only one deck slot needed for the swap.