Step 1: Your Deck
Make sure that you have some offensive as well as defensive cards. Offensive cards usually have less hit points, or go straight for crown towers without attacking other troops along the way. Defensive troops have more hitpoints, and usually do splash damage or concentrated bursts.
Make sure you look at the average elixir cost. You don't want a deck that has an average cost of 5, or you won't be able to deploy enough and you'll get overwhelmed. Again, look for a balance. A well balanced deck should cost between 3.5-4.8 elixir on average.
Step 2: Counter Attacks
Skeleton Army- counter with valkyrie, minions, or baby dragons.
Prince/Knight/Hog Rider/Giant- counter with skeleton army.
Baby Dragon/Balloon- counter with minion horde, spear goblins, or archers.
Giant Skeleton- counter with minion horde.
Any large quantity of weak troops- counter with arrows.
Valkyrie- counter with minions
Witch- counter with prince directly behind it.
Buildings- counter with rocket or fireball.
Step 3: Offense
Good combos can destroy crown towers quickly, if done right. Here's how.
Deploy two troops. A prince in front of a witch works very well. A giant in front of a balloon protects the balloon from damage and allows it to destroy the crown towers. Experiment and find your own combos to win!
Giant and Balloon
Witch and Prince
Goblin Barrel and Freeze Spell
Skeleton Army and Spear Goblins
Minions, Minions Horde, and Rage
XBOW and Rage Spell
Step 4: All Done!
That wraps up this instructable, please vote and comment if you like it.