Currently, single-player is the only way to earn new cards through gameplay. You earn points for actions during duels (e.g., successful summons, high damage); every 20 points awards you one card (up to 15 per match).
The game is split between a challenging single-player campaign and various online multiplayer options:
Because card unlocking is entirely random, many players recommend purchasing specific DLC Card Packs (approx. $1.99–$2.99) to build a competitive deck quickly.