If youll are having trouble beating a specific unit, look at their element I will list them. Fire Beats Wind, Wind Beats Earth, Earth beats Thunder, Thunder beats Water, Water Beats Fire. Thats the cycle.
Ohh and just so youll know your units will deal more damage to an element it is strong to and take less damage, If both units are same element they will both take half of total damage and deal half of total damage, if your units are weak to a unit element your units will deal X.2 damage and recieve more from the enemy. So make sure to get a variety of units with diffrent elements!
+ This so everyone can see
 "SAS: Zombie Assault 3"){kind=icon}
 "Epic War Saga"){kind=icon}
 "Bloons TD 5"){kind=icon}