Below are some general tips to do better in Card Wars.
Try to complete as many quest challenges as possible or participate in Deck Wars - you may win gems. You may also be lucky and receive gems from the holiday gifts. Obtaining all 3 stars will earn you a gem. Save these gems for opening the Algebraic Chest to get rare and powerful cards.Additionally, take advantage of Card Wars Carnivals to obtain the cards you want.
However, when trying to three-star quests for gems, it can be a good idea to check the Quests page to make sure that all the quests are accomplishable. For example, there is no point in wasting hearts completing two of the quests if the third is something you can't do - e.g. if you played an all Corn deck and the quest was 'Beat the opponent using only Useless Swamp cards'
If you want to continue battling and are getting low on hearts, there are three options. The first option is to wait for your timer to hit 0, the second is to use a gem, and the third option requires beating certain quests. This will give you an additional heart and restore your hearts completely. These quests are the third appearance of the same character in a row. They will have a larger circle icon on the map. You can always beat the first two versions of the character and save the third battle for when you need to restore hearts.
Generally, it is a better decision to try to go first in a match due to the magic point advantage.
Depending on your deck strategy, the way in which you spend magic points is different. At the start, make sure most of your magic points go into placing your creatures. Later on, floop abilities should be considered. Spells are better used when absolutely necessary, as most cost a lot of magic points. Plan your magic points to best usage before attacking.
It is good to get some damage in on empty lanes with powerful level 1 - 3 creatures with high attack or block up lanes with high defense, low magic cost creatures
Constantly check on your opponent's and your Hero abilities. Keep in mind how many more turns it will take before you or your opponent can activate it as it would affect the course of the game.
Always plan ahead. What you do this turn will affect the next. This ties in with the previous two points as it is important to keep track of your Hero ability, note your magic points as well as know your hand. Consider all possible actions and evaluate them, taking into account how your actions during your next turn will be affected, for example if you should spend your magic points this turn to recycle a powerful card, or delay that and play other lower cost cards this turn, since you can tap on your Hero ability next turn for an added advantage.
Avoid attacking with low defense creatures and leaving your hand empty as you will be susceptible to open lane devastation from spells and floop abilities.
If you draw a card later in the game that has a lot of attack but the lane you hope to place it on already has a creature and a creature is in the opposing lane, don't replace it immediately. Your opponent computer is not programmed to recognize possible replacements. Make them leave lanes open with your weak creature in it and replace it with a heavy hitter.
Going Second Strategy
While at a lower level, going second is often a sure fire way to take the lead and ensure victory. If You do you can kill what they play easily or hit them directly, and if you play a lot of 1 cost cards redeem the brief power card with its code, and you can load most if not all your field on your first turn.