Add a whole bunch of troopers to your defense, whether they’re novice, experienced or better (depending on your level). Hoards of troopers with no leader are a HUGE pain to take out; clustered together they are even moreso because they’ll take out large numbers of the other player’s attack troops. Plus, they are very cheap to add (only 5 points required for the novice troopers).
Once you unlock the turrets, they and the troopers make a deadly combination together. Turrets take forever to take out and will constantly harass the attacker, making it a huge ordeal for them to take over just one tiny little strategic area. Other variations on them, such as the turret tanks, are equally effective against attackers.
When you are the attacker and you get faced with a turret that’s right next to a strategic area, take out the turret BEFORE you try to take the area. None of the enemy’s hoards of defenders will come out until you until after the area is taken, so pepper the turret with shots, draw its fire and run out of the way, then shoot it again until you’ve destroyed it.
If you want to earn experience extremely quickly, look for daily achievements to come up that give you experience points as a reward for unlocking them. They won’t always show up but when they do, you may earn several hundred or even thousands of EXP as a reward for completing them. All of that EXP can cause you to gain multiple levels at a time, so when you unlock one of these achievements, immediately check out what new upgrades are available.
Want to earn medals and make big combos? If you’re surrounded, keep your formation closed in together. If enemy leaders are all lined up, then draw a line parallel to the imaginary line between the enemy leaders, so you can blow them away all at once without having to deal with leader bubbles popping. Tap the bubbles in short succession to make larger combos and earn more medals.
The combo size and medals earned are crucial to earning a higher grade for a battle, but make sure that you don’t jeopardize your survival in pursuit of the really good rankings. The highest rankings you can get are S, followed by A, and the higher the letter rank, the more experience and coins you will earn as a reward for winning the battle.