This may appear as daunting at first. Especially with characters like Scarlet Witch/Nico due to them not being as strong.
But you actually require very little effort in order to build better decks. You can also modify the cards for them to offer bonuses or extra damage.
You can also use the same attack over and over as it’s guaranteed to re-roll on the card deck if you only use that one + if each use equals a KO (ie: Hex bolt for Scarlet Witch, it starts with 2 chains but scales up to 5 chains, guarantees a KO at max chain streak).
You can also use Agatha’s Cauldron to create “Vial of Shadows”, which gives concealed for a turn, you can then end the turn and nothing will attack you. He’s to use this on the third and last turn. Consumables do not get used up, so you can skip the third turn for every character using this method.
Doctor Strange, Blade and Hunter also have cards with “Concealed” stats on them, so these three will be super trivial compared to the rest. As for the rest, make use of high damage, knockbacks, forceful knockback etc. You’ll get it in no time if you simply upgrade all your cards and modify them.