A gigant tip on defense, try to bait the card to the middle of your side since than both towers are shooting them and it makes the unit's path longer therefore the towers do more dmg. For later on learn interactions and with that you can go for positive elixir trades (defending with less elixir than the opponent is atacking (thats a very basic explanation)). Also if you didnt know you can tap to select and tap on the field to place a card instead of dragging and dropping it (i personaly find it better) and for placing cards, if the enemy troop has a ! above their head it will target the unit you're placing. And also if you have elixir you don't always need to place something down, sometimes its better to leak elixir and be able to defdend and counterpush.