In Rage of Bahamut your attack deck is your bread and butter. In my Rage of Bahamut Guide I cover several things as a generality for beginners. In this portion I will be focused on Attack Decks and the importance of them.
To begin, the core of your attack deck is determined by your attack points. Every card has a cost attributed to it. Your attack decks total cost can not exceed your number of attack points. An example of this would be if you had 100 attack points. You would not be able to have five 22pt cards in that deck since the total cost would be 110. You could have three 22 point cards for a total of 66 then two 17pt cards for a total of 34 to equal a perfect 100.
In your attack deck you should have your cards with the highest attack power that you can achieve. If you use the recommended deck option, the game will automatically figure this for you.
Another part that plays into your attack deck are the skills that your cards bring to the table. Cards will have a variety of skills. The skills will focus on either boosting your cards or applying a debuff to the opponents cards. The buffs you want to look for in your attack deck are Increase to ATK, Increase to ATK/DEF, Decrease to enemy DEF, Decrease to enemy ATK/DEF. Your cards skills won't always trigger, so many attack decks are stacked with five cards with skills to ensure the maximum chance of them triggering. Also, there are skills that focus on cards of a specific alignment, be aware that you want to attempt to stack your alignment to get maximum benefits from these cards.
Attacking plays the role in Holy Wars of gathering points, it is also a method in which to attain treasure. You can attack other players to receive their treasure and up to 10000 rupies based on how much currency they currently have.
Later in the game having 220 attack points you'll be able to attack with five 22pt card deck twice in a row. 330 three times in a row. Your attack points recharge over time or you can use Holy Powder to instantly recharge them.
As you are leveling and not creating your final cards remember that leveling five cards to level 10 in your deck through enhancement is better than leveling one card to max. The fastest levels to attain on a card are the first 10-15, so get five decent cards there then you can focus on increasing them.