Defending is the key to victory. Just having a good attack isn’t good enough to secure you the victory. You need to have a balance of both attacking and defending! This is just a general guide on how to defend in Clash Royale. If you want to know how to defend against a specific card, you can view it at the cards itself!
Having a balance in your deck. Having just offensive troops, is not going to help you defensively. You will not be able to defend efficiently. Which may in turn, affect your offense!
Have single-target damage and splash damage. Single-target damage can be used to destroy tanks/other single-target damage troops while the splash damage is able to help you destroy swarms easily.
Placement of your building cards. The building cards, should be placed in the center of your base, unless it is a spawner and you want to maximize its spawns. By placing it in the middle, both towers are able to attack the troop, hence, you will deal even more damage.
Placement of troops. If your troops are squishy, you would not want them to receive damage. Hence, you would not want to place them at an area where an ! appears.
Know what card counters what. You would not want to use a Prince to defend against a group of swarms and Barbarians. Your Prince will just get destroyed. Hence, you can use the Bomber or the Wizard. This also prevents you from wasting Elixir on defending, as you are able to destroy your opponent’s attack efficiently.
Elixir control. Using all your elixir on the offensive end, may not be beneficial for you as your opponent can launch a counter attack on you, which can deal even more damage.
Damage spells. Damage spells such as the Fireball or Zap can be used to help you clear swarms or semi-tanks.
Do not hesitate to use your troops etc. Sometimes, players tend to hesitate and risk to see if their troops are strong enough. If you have placed your supporting troops earlier, you might have prevented your troops from dying or your tower from being damaged.
Taking damage at times is okay. Rather than wasting your elixir to just destroy a troop that will probably only attack your tower twice, is fine.
Luring. The art of luring is very important as it can prevent glass cannon troops from attacking your tower or your own glass cannon troops from getting targeted. Here’s a video on the different ways you can lure troops!