Can be earned naturally during the Story Mission “All The Devils Are Here”. This is at some point late into the Southwest Story Missions (Act II / Map 2).
I won’t spoil the story content. But the encounter itself is basically an “onslaught” encounter and upon every KO you make, you are guaranteed a card refund. So you can activate to your heart’s content so long as you get a KO each time, it’s really easy, as a lot of the enemies are the standard weak ones, knocking them into the chunkier ones makes it even better.
As always, make real use of Knockback and Forceful cards. Hero cards are the ones that cost Heroism, as you get KO’s, you’ll get 1 Heroism per. That’s it, happy hunting!