How do I level up useless cards?

There are cards like Misty Knight, Abomination or Shocker, that I will never play.

Is there a way to level them that doesn't involve just throwing them all in a deck and losing games?

Outside of waiting for fast levels to appear in the shop, i think you really do just have to stick em in decks. . . at least Abomination has a 9, the other two are just awful, haha

You can get boosters for them randomly from reward chests as well.

Yeah I think just swapping them into a deck for a little while is the way to go. Luckily you only have to get that cheaper intital boost for them, and after that the cost of upgrading is relatively the same across upgrade levels (100 credits per level).

Also remember that even though cards with no special abilities are pretty underwhelming for the first 2 pools, in pool 3 Patriot exists, which gives them some new life.

With the new patch, post-game Boosters will favor cards in your deck for which you currently own fewer than 20 Boosters; so if you've got a deck with a lot of high-upgraded cards you can swap in one of those underused cards and have a better chance for getting Boosters for it.

Generally speaking, it's only more efficient to upgrade to Uncommon (25/5 for 1 CL) - every other upgrade gives the exact same Collection Level boost for the amount of Credits/Boosters spent (50/10 for 1 CL). So once you've got them to Uncommon there's not too much reason to keep grinding for their Boosters if you don't have a deck that wants to play them.