Here are a few basic techniques that might be able to help you in your game:
1. Heroes/ Units Shuffling
Sometimes you might not like to have 2 or even 3 spear wielding heroes in your starting party or do not want to restart, but enemies are too strong and coming from many directions. Just sell off your spear wielding hero, and plant him on the next location, rinse and repeat. This is especially helpful in Loop 2 games. If you intend to do this, make sure you always keep like 50 to 100 gold before selling off the unit that you intend to re-plant.
2. Remember to replace units
This will help tremendously in Loop 2 games. Example if you have observed that aerial units will not appear anymore after wave 10, just sell off all your archer units and replace them with stronger ground troops, and vice versa.
3. Choose your kill zone
When enemies are coming from many directions, choose a location where they will converge as your killing zone. When place a unit, the best location is where it can at least hit 2 paths. Sometimes the path is not obvious (especially spear path), just pause the game and look around.
4. Placing your spearman / hero
Always place the spearman (hero) on a path that is facing the enemies instead of on a path that the enemies are moving away. It is more effective and will score much more kills.