Know your Enemies
You will soon discover that you can only get so far by just spamming your enemy with attacks and that to continue winning battles you will need to maximise each wave. To be able to do this will mean knowing who your foe is beforehand so you can decide what units to use. This can be done by clicking the skull icon which will tell you what kind of enemy you can expect to see next. The first time you encounter an enemy the game will also give you some useful information about them.
As you progress through the game defeating different types of enemies you will eventually struggle to keep track of their strengths and weaknesses. When you reach this point you can simply tap on the enemy unit to bring up the information you require. Get into the habit of reviewing enemy information before placing down new towers so you know what foes are nearby.
Always look for a strategic advantage when placing down your units. If possible try and find spots where units can work together in order to overwhelm the enemy. An effective tactic is to place your meleee barracks near your ranged towers so they can deal with enemies that get through the towers ranged attack.
Keep an Eye on Upgrades
Make sure you keep an eye on upgrades so that you can stay on top of them. There are two types of upgrades in the game, those for upgrading your heroes abilities and those for uprading your power ups. Upgrading your heroe's abilities cost Hero Points which you acquire by leveling up your heroes and upgrading your power ups costs Souls which are acquired by progressing through the campaign stages. It is recommended that you focus on purchasing upgrades in the second tier so you can then use up your Souls for the first, third, and fourth tiers. Pick just one of the upgrades in a given tier before shifting your attention to the next one, doing this will help you from focusing too much in a single tier. The most important thing that you spread your upgrades out so you have a well-rounded team.
Time your Upgrades
An upgraded unit will instantly enter the battlefield whenever you upgrade a tower. If you can time an upgrade to coincide with the inital wave you send out so you have two waves entering the battlefield in quick succession you will find in most cases the enemy will have difficulty dealing with both waves at once. The best time to use this move is in the later parts of a stage in order to secure your victory.
Power ups are purchased with gems and the maximum number you can bring to the battlefield is three, this means you need to make sure you don't waste them. The type of power ups you can take is entirely up to you but it is advised that you take two damage-dealing ones and one that gives you a stat boost. The most beneficial time to use power ups is in the later parts of a stage when you encounter powerful enemy waves.
Completing the various achievements in the game is the best way to get more gems. These are milestones in the game which when you reach will reward you gems, the more difficult the milestone is to reach the more gems you will earn. The tasks in these achievements vary and include things like reaching a specific level and killing a certain number of enemies. It's always worth checking the list of achievements as there may be one you are near to completing which you can then focus on to get the reward.
When you have beaten a stage for the first time you will unlock the Iron and Heroic version of the stage which will earn you substantially more gems if you can beat them. In Iron mode you must defeat one Elite Wave with just one life and in Heroic mode you must overcome several Elite waves with just one life. The only difference in these modes is that you can only use the pre-set tower setups.
If you have towers in positions that are no longer useful you may want to consider selling them. Although you face a gold loss selling them there will be times when doing so is the best course of action as it allows you to invest in a new one which you can place in a strategic position that can change the tide of battle. A good time to do this is when you reach the final waves of the game when you find yourself struggling to contain the enemy.