It's not a one time thing, it's a daily quest where you have a 24 hours from the first win (not sure) to get 20 wins for the epic chest, after that you can keep playing and get regular chests. The SinglePlayer is only good to teach you the basics and most of the cards before you hopefully move to MP (you'll have your share of OMFG moments in multiplayer from Rare cards).
Got really rustled today after losing ~10 matches in a row and dropping from 1306 to 1056 rating. The flavor of the day is frenzy stacking on your warr/mage and one shotting shit. Also, fuck fireballs, had a fight against a guy with unholy power (+4 damage) and 2 fireballs in round 1, my whole team was at 30% hp by round 3.
I feel they need to tweak the damage on some cards (and remove a lot of frenzy type buffs that each gives between 10% to 400% bonus damage), and either allow you to draw 3 cards per round and/or add a guaranteed attack/magic/heal card just like the movement one.
Oh, and after 200-300 matches in MP and lvl 12 in SP I have bought 2 items from the shops.