It is essential that your cards be positioned correctly. There are two rows; cards in the first row will be attacked first. Place "tank" cards (high health, marked by a shield icon) at the front. Make sure your best "tank" card is in the middle, as most Bosses often appear at that location, where they deal damage to the card directly in front of them. Position cards with lower health, such as ranged units and healers, in the lower row.