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.
Einherjar
Complete all maps to unlock the Einhenjar feature. This is an extra bonus that is used automatically while in battle. You can either spend coins to get the map pieces faster (at the "Research" tab) or by playing the map levels repeadly. You can deconstruct existing Einherjars to get materials required to upgrade the ones you have. You also have a daily chance to randomly improve their skill by tapping the "Update" button at the menu.
Evolution
Different equipment cards are required to evolve a card. If needed, grind previously completed levels to get them. When you get cards that can be used for to evolve the ones you already have, replay those levels.
Log in bonuses
Log in daily for the first seven days of playing to have a chance of obtaining premium currency, rare cards, and resources.
Upgrades
Level your best cards, especially the leader, first. Only level rarer cards that will be kept over time.