A you level up and win extra cards, choose your best cards locked so you do not use them by mistake and only level them up. Leveling a card costs money that should not be wasted that on a card that you will stop using soon. Usually, the more stars a card has, the better it is for your squad (especially because it has a higher maximum level) As soon as reach maximum level with a card, you can evolve it if you have the required cards. Evolve your cards as soon as possible to give them more power and room to grow.
Coins
Farm previously completed battles to earn coins. Besides using cards to enhance or evolve, you can sell them for coins or just to make room for others. Earn achievements to get rewards that include special cards, gems, and extra coins.
Combos
Attack with combos in mind when possible. Because your selected gem will move all the others on the level, create combos. Find a match by moving a single gem, and keeping the other gem available for dragging around. If you do not have a complete deck, it indicates that some gems will not trigger an attack. Use this as an advantage by using these gems in combos.
Decks
Try to build a deck with one of each of soldier type (red, green, blue, yellow and purple), so that every match made will trigger an attack. However, this is not always the best strategy because you can select your ally from the missing color or use the bonuses of your leader to increase the stats and attack power of your dominant color. Red beats green, green beats blue, blue beats red, and yellow and purple beat each other.
Health
If you match health packs while you have maximum health, that health boost will be wasted. Try to keep the health packs ready but only use them when really needed. Use them later when the enemies are stronger.
Leader cards
Some cards can be "leaders", resulting in various different bonuses (for example, reducing damage from a specific type of unit, or extra hit power). Examine your cards' leader skills and assign the card that helps you the most as leader. You can still keep your previous leader in the squad.
Special skills
After a specific number of rounds, some of your cards will gain the option of triggering a special skill. Enabling those skill only when needed.
Squad capacity
You start with 23 "power" for your squad. Expand this capacity by spending diamonds in the store, at the cost of two diamonds per unit of power. More squad capacity allows you to have higher level troops in your squad.
Cards...