Originally Posted by
Fwahm
It seems you get one card for every five objectives you clear, and it carries over between Omen runs. This counts the optional objectives, like WS 30 times in 90 seconds, and the objectives to unlock the next area.
For example, if you clear 2 optional objectives and make it to the Thinker, but wipe, that's 4 objectives done (2 optional, 2 floor unlocking). When you next enter Omen, you'll get a card when you complete another objective, and it'll reset.
When going in solo, I have been getting 5-6 total objectives done each run, which is slightly more than a card on average. If you don't have a group that can at least clear the mid-bosses, going in solo is better for card farming.